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© Raymond Pettibon 


RESEARCH LIBRARY 
eee ey ry RESEARCH INSTITUTE 


JOHN MOORE ANDREAS COLOR CHEMISTRY LIBRARY FOUNDATION 





INTERNATIONAL CHEMICAL SERIES 
H. P. TALBOT, Pu.D., Sc.D., Consuztine Eprror 


THE HYDROUS OXIDES 





INTERNATIONAL CHEMICAL SERIES 


(H. P. Tatsor, Px.D., Sc.D., Consuttina EpiTor) 


Bancroft— 
APPLIED COLLOID CHEM- 
ISTRY 
Second Edition 


Bingham— 
FLUIDITY AND PLASTICITY 


Cady— 
INORGANIC CHEMISTRY 


Cady— 
GENERAL CHEMISTRY 
Second Edition 
Grifin— 
TECHNICAL METHODS OF 
ANALYSIS 
As Employed in the Labora- 
tories of Arthur D. Little, Inc. 


Hall and Williams— 

CHEMICAL AND METALLO- 
GRAPHIC EXAMINATION 
OF IRON, STEEL AND 
BRASS 


Hamilton and Simpson— 
CALCULATIONS OF QUAN- 
TITATIVE CHEMICAL 
ANALYSIS 


Loeb— 

PROTEINS AND THE 
THEORY OF COLLOIDAL 
BEHAVIOR 

Second Edition 


Lord and Demorest— 
cat EN prarte oct ar ANALY- 


SI 
Fifth Edition 


Mahin— 
QUANTITATIVE ANALYSIS 
Third Edition 


Mahin and Carr— 
QUANTITATIVE AGRICUL- 
TURAL ANALYSIS 


Millard— 
PHYSICAL CHEMISTRY FOR 
COLLEGES 
Second Edition 


Moore— 
HISTORY OF CHEMISTRY 


Norris— 

TEXTBOOK OF INORGANIC 
CHEMISTRY FOR COL- 
LEGES 

Norris and Mark— 

LABORATORY EXERCISES 

eae EE eda CHEMIS- 


Norris— 
ORGANIC CHEMISTRY 
Second Edition 


Norris— 
EXPERIMENTAL ORGANIC 
CHEMISTRY 
Second Edition 


Parr— 

ANALYSIS OF FUEL, GAS, 
WATER AND LUBRICANTS 

Third Edition 


Robinson— 
THE ELEMENTS OF FRAC- 
TIONAL DISTILLATION 


White— 
TECHNICAL GAS AND FUEL 
ANALYSIS 
Second Edition 


Williams— 
PRINCIPLES OF METALLO- 
GRAPHY 


W oodman— 
FOOD ANALYSIS 
Second Edition 


Long and Anderson— 
CHEMICAL CALCULATIONS 


Bogue— 

THE THEORY AND APPLI- 
CATION OF COLLOIDAL 
BEHAVIOR 

Two Volumes 


Reedy— 
ELEMENTARY QUALITA- 
TIVE ANALYSIS FOR 
COLLEGE STUDENTS 


Leighou— 
CHEMISTRY OF ENGINEER- 
ING MATERIALS 
Second Edition 


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PRACTICE OF ORGANIC 
CHEMISTRY 


Eucken, Jette and LaMer— 
FUNDAMENTALS OF PHY- 
SICAL CHEMISTRY 


' Underwood— 


PROBLEMS IN ORGANIC 
CHEMISTRY 


Schorger— 
THE CHEMISTRY OF CELLU- 
LOSE AND WOOD 


W eiser— 
THE HYDROUS OXIDES 





THE HYDROUS OXIDES 


BY 
HARRY BOYER WEISER 


Professor of Chemistry at the Rice Institute 


First EDITION 


McGRAW-HILL BOOK COMPANY, Inc. 
NEW YORK: 370 SEVENTH AVENUE 
LONDON: 6 & 8 BOUVERIE ST., E. C. 4 
1926 


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4 
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yi 
ij 44 


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ptt 


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CopyriGHT, 1926, BY THE 


McGraw-Hitt Book Company, 





PRINTED IN THE UNITED STATES OF 


THE MAPLE PRESS. COMPANY, YO! 


THE GETTY RESEAR 





PREFACE 


The scientific foundation of modern colloid chemistry was laid 
by Thomas Graham more than three score years ago as a result 
of his basic researches on the colloidal behavior of albumin, gums, 
and gelatin, and of the hydrous oxides of silicon, iron, aluminum, 
chromium, tin, titanium, molybdenum, and tungsten. Since 
Graham’s time a great many investigators, van Bemmelen in 
particular, have studied the colloidal character and application of 
the hydrous oxides. So far as the author is aware, the present 
volume represents the first endeavor to correlate systematically 
and summarize critically the numerous scattered facts in an old 
but increasingly important field. 

No group of substances presents a greater variety of colloidal 
properties than the hydrous oxides. For this reason they have 
been employed frequently in the investigation of colloid chemical 
phenomena and applied in widely diversified ways to the indus- 
trial arts. There is little doubt that a more intimate acquaint- 
anceship with this group of substances will serve to extend their 
field of usefulness rapidly. It is hoped, therefore, that the book 
may prove of value alike to scientist and industrialist. 

Portions of the manuscript of the book have been read and 
criticized by several gentlemen. Special acknowledgment of this 
sort is gratefully made to W. D. Bancroft of Cornell University, 
k. H. Bogue of the Bureau of Standards, F. L. Browne of the 
United States Forest Products Laboratory, E. M. Chamot of 
Cornell University, P. L. Gile of the U. S. Department of Agri- 
culture, and C. L. Parsons, Secretary of the American Chemical 
Society. 

Harry B. WEISER. 


Houston, TEXAS. 
Feb. 1, 1926 


. 





CONTENTS 


PAGE 

Rete eM a oe lw ke we te Vv 
ERTRODUCTION 2.1... .. Oh Or EN oe oy Fae rn ee es 1 

CHAPTER I 

JELLIES AND GELATINOUS PRECIPITATES ............. 33 
Se ne 3 
IC etre dO rs dg So a Sack bok ook ein oaks 15 
DOMME CUTO CLE TIONS fF)... ous oo ade as ve be bbe cee vbedces 30 


CHAPTER II 


MeEMSSSIPOURTORIDNS OF IRON... . ... :. .. ss . sw oe 34 


Bette eer OXIIG ka ecm se ccd sense nu sbveuecta 34 
TOT er yj, occas ac ale cash bw dpe bas 38 
The Precipitation of Sols by Electrolytes................... 55 
Basorpuon py tHydrous Ferric Oxide.........0...00e 260s ae 67 
Pret morornyarous Perric Oxide... ........:.6.c00.00ees 70 
I COR ORT OT os id vs ema s Pisa eens gcaleone es 74 


CHAPTER III 


PeITIPRCCHOMICHAIXIDE: . 2 6k he ek eee SB 


Pea RU RRO a shee eae wg d SP alate we Pelee mela os 82 

Pree PUGUAOSOIPtlON 2 oo. dees cde ca eee cave wb aleas 91 
CHAPTER IV 

Tue Hyprovs OxiIpEs oF ALUMINUM, GALLIUM, INDIUM, AND THALLIUM 103 

emer epee INU, CXICG sy f505).) cine ca Gls cds vies we coe ee ee ae 103 

De BReNMTH ERR ULE DIOLS kD, oy ae Pode dis RY « vay wd Sea 112 

Adsorption by Hydrous Aluminum Oxide.................. 122 

IKI, ho 1G. es dinin cool pin woe dw cay ne wie pape Par oek 129 

RE MMEPMTNOE  XIC OS cs fs kl we 225 oie 4 spare W Oa arlene a vm 131 

MMR EOC INUCLG Ca yh sa cas wpe glace a abeniinn SNe eee 132 

CHAPTER V 

Ture Hyprovus OxipEs or Coprmr, CoBALt, NICKEL, SILVER, AND GOLD 134 

Dee mrTeme Re CYKICG 2 i-5 eer excel ots os ft Da enw es es awe ee 134 

Piste AEUIDIOUS, OXIGG . ies se ee acsce eles Hae wee OS Pi a 145 


vill CONTENTS 


PAGE 
Hydrous Cobaltous Oxide...........0. 45... «s/o 147 
Hydrous Cobaltic Oxide... ...V.css. 0.0 @.- . <r 151 
Hydrous Nickelous Oxide.....+........+.00.5 0%. 152 
Hydrous Nickelic Oxide and Nickel Peroxide........-.2. 7.7) ow 
Hydrous Silver Oxide.:......s 11.22... 05.008). 0s 156 
The Hydrous Oxides of Golds... 0s. =... . 156 

CHAPTER VI 
THE Hyprovus OxipEs or BerytirumM, Maagnestum, Zinc, CaADMIuM, 

AND MERCURY... 2000606 2 els 3 5 op 
Hydrous Beryllium Oxide......,..- .) 0...) 159 
Hydrous Magnesium Hydroxide... ...... 2). ee 164 
Hydrous Zinc Oxide. ... oo. 0 bees ds 4 oe 169 
Hydrous Cadmium Oxide... .:..... 0755 ee 172 
Hydrous Oxides of Mercury....,0..,... > . 0 er 173 


CHAPTER VII 


Tue Hyprovus OxIpEs oF SILICON AND GERMANIUM. ....... 175 


Hydrous Silicon Dioxide. ............2.:05.5)) one 175 
Silica Gel... Oo ee ates we oe lo 175 
Silica Sols... 06.0.6 ssc 0 ge ae ws pw on cis oe Ogee Seen 193 
Silicate of Soda... 2... 0.2 vn sm om dee 6 een 196 

The Hydrous Oxides of Germanium...:>..... 20.56) 199 


CHAPTER VIII 


Tue Hyprovus Oxiprs or Tin anp LEAb, 2 eee Me 
Hydrous Stannic Oxide.:......0.1....... ©, ae 202 
Precipitated Hydrous Stannic Oxide... ., 7-235 202 

Stannic Oxide Sols... 2... 0655 <2 1-6 2 215 
Hydrous Stannous Oxide....../.: 2.2... «0 |e 224 
Hydrous Lead Monoxide...... Fr aes re 225 
Hydrous Lead Peroxide.....: 2.0...» «4 5: 230 

CHAPTER IX 

THE Hyprovus OxIpEs OF TITANIUM, ZIRCONIUM, AND THORIUM . . . 233 
Hydrous Titanium Dioxide... .°.....25.. 772) Pee 2 
Other Oxides of Titanium.......*....:..0...50 236 
Hydrous Zirconium Dioxide. :...........-. 5) eee 237 
Zirconium Dioxide Sols... .... 2... 1.0... 5 vee 241 
Adsorption by Hydrous Zirconia. ....../ 244 
Hydrous Zirconium Peroxide. ......... s <u: eee 245 
Hydrous Thorium Dioxide.....<2..... .. =. een 246 


Hydrous ‘Thorium Peroxide... 5... .. «+ <0) see 250 


CONTENTS 
CHAPTER X 


THE Hyprovus Oxipges oF THE Rare Eartus. ... . Reena 
The Hydrous Oxides of the Cerium Family................. 


The Hydrous Oxides of the Terbium Family............... | . : 
The Hydrous Oxides of the Yttrium Famiily................... 


CHAPTER XI 


Tue Hyprous OxIpDEs oF THE FirrH GRovUP. 


Petey MCT POENLOXICE. . 2... occ ak lee ee te ee ee ee 
Breet F ONLOXIGE DOIS., 6... eee ee cine oe meee ees 
Beem rts Of VANROGIUM, .. 6... fo ee ee nee eee een 
Papttemm Olumbinm Pentoxide..:.......5... 00020... Dee ce et 
Sa ee a NIN FCNUOXICE.. 0... ee ee ec eet 


’ The Hydrous Oxides of Antimony......... 


MMe etiot Bigmith:cises.... sco icc sedeevcles. 


CHAPTER XII 


Tue Hyprovus OxipEs oF MoLysBpENuM, TUNGSTEN, AND URANIUM. . 
eet yaroueoxiges of Molybdenum, 0. i... cde ee ee ee bles 
Baten Were Oxides Of Lungstene i... oo we ee ede ee len 


ThesHvdrous Oxides of Uranium.:............, 2.006.000 0. 
CHAPTER XIII 


Tue Hyprovus OxipEs oF MANGANESE. . . 


DRS CAVE SEN EEG GH. CON D0). 4 06 (0 ra 
Romer Wyorous Oxides of Manganese... ..... 5.6.6.0 


CHAPTER XIV 


Tur Hyprovus OXIDES OF THE PLATINUM FAMILY. . 


memivdrous Oxides of Ruthenium. ..:......5....s2cee Fees 

emetyaroticscicos of Rhodium... 0... 60.6 ok ee Se ae 
mea yurous.(xices of Palladium... .2..¢.0.7.. 6h seat a eee. 
Pee yrour Cries Of OSTIUM oo... Ge wee ne ee ee ek ee ee 
Mr arora mies Of ITICMUIh so. cere so xn bs ee oe Owe EE Son 
Meet ytoue (xides of Platinum ..- 2... 6.0.2. ee eyo eee ee cue 


CHAPTER XV 


TANNING . . : 


REM SONS es act ite tan ah Py ah Oe atl © he Dad ROE 
RM ie nics i> co, api td 4 ee eet aoe aaa ee 


MORDANTS..... . 


er ererin creme ett ee PN Tee oe es cc uics ah alah Daa eee 


1X 


x CONTENTS 


Tannin » rec in oss wisic fhe cco ee ee 
Fixing Agents. ... . oss <4. < > <s.s9 ew ee wom eels ee oar ee 
Color Lakes}. iss. 6. buon be ce ee ne ee 


CHAPTER XVII 


WATER PURIFICATION. 


CEMENT. ... ! 


Portland Cement.:...5 6.044% ¢0e¢1 be es 


Relation between Properties and Composition of Soil Colloids... . 
The Réle of the Soil, Colloids: .:..... 2...) 


Supsecr InpEX..> 2... 


THE HYDROUS OXIDES 
INTRODUCTION 


When a solution of a ferric salt is treated with an alkali, there is. 
formed a voluminous, gelatinous precipitate which is commonly 
called ferric hydroxide and assigned the formula Fe(OH)s. 
The extent to which this terminology is fixed in our chemical 
literature is evidenced by its almost universal use in our text- 
books, although four decades ago van Bemmelen! showed not 
only that there is no definite hydrate of the formula Fe(OH); 
or Fe.03:3H.O, but that no other hydrate is formed by the usual 
method of precipitating the oxide. The viscous voluminous 
precipitate when first formed may be represented approximately 
by the formula Fe.03;- +2O0H,O, but it loses water, gradually 
attaining a composition that varies with the time, the tempera- 
ture, and the pressure of the water vapor in contact with it. A 
composition corresponding to a definite hydrate is, therefore, 
purely accidental, depending as it does on the exact method of 
formation, the method of drying, the temperature, and the age of 
the sample. Precipitated oxides like ferric oxide which contain 
varying amounts of water adsorbed by the oxide particles are 
called hydrous oxides to distinguish them from hydrates, in which 
the water is chemically combined in definite stoichiometric 
proportions. There are a few hydrated oxides, such as Al.QOs- 
3H,O and BeO:H;0, which may adsorb varying amounts of 
water, depending on the conditions of formation. Such prepara- 
tions may be termed hydrous hydrated oxides. On standing, the 
primary colloidal particles of the hydrous oxides grow and lose 
water spontaneously, causing the mass to assume a less gelatinous 
and more granular character. This spontaneous transformation 
from a loose voluminous precipitate to a granular mass is accom- 
panied by a decrease in the solubility, the adsorbability, and the 
peptizability of the compounds. 


1 Rec. trav. chim., 7, 106 (1888). 
1 


2 THE HYDROUS OXIDES 


Although the rapid precipitation of a hydrous oxide usually 
gives a gelatinous mass with a supernatant liquid, it is frequently 
possible to bring about uniform precipitation throughout the 
entire solution with the formation of a jelly which differs from a 
gelatinous precipitate in that all the liquid is enclosed by the 
precipitated phase. Since the hydrous oxides are obtained so 
frequently in the form of gelatinous precipitates, and since the 
latter are produced when jellies contract spontaneously or are 
broken up by stirring, the first chapter deals in a general way 
with the structure, preparation, and properties of gels. This is 
followed by separate chapters devoted to the typical oxides of 
iron, chromium, and aluminum, after which the remaining 
oxides are taken up by families in the approximate order in which 
the elements appear in the periodic table. The last five chapters 
are concerned with some of the more important industrial applica- 
tions of the hydrous oxides. 


CHAPTER I 
JELLIES AND GELATINOUS PRECIPITATES 


Gelatinous precipitates and jellies are the two forms of solid 
or semisolid colloids that are commonly included under the term 
gel. Gels of the hydrous oxides, such as ferric oxide and chromic 
oxide, which lose their elasticity and become powdery on drying, 
are called rigid or non-elastic gels in contradistinction to the 
elastic gels, such as gelatin, albumin, and agar, which are charac- 
terized by perfect elasticity through certain narrow limits and by 
retaining their elasticity and coherence on drying. Although a 
detailed discussion of the properties of elastic gels les beyond 
the scope of this book, any adequate theory of gels must take them 
into account. Moreover, the vast majority of the work on gel 
structure has been done with gelatin, and a survey of the results 
of these investigations throws considerable light on the nature of 
gels of the hydrous oxides. 


STRUCTURE 


Since a working theory of the structure of gels is necessary for a 
systematic discussion of their preparation and properties, we 
shall take up first the question of the structure of the two forms, 
beginning with jellies. This question has doubtless received 
more attention at the hands of investigators than any other 
single problem in the field of colloid chemistry; but, in spite of 
this, opinions differ as to the exact nature of a jelly. Thus 
Robertson, Procter,! and Katz? regard jellies as homogeneous 
single-phase systems, solid solutions, or semisolid solutions “of 
the exterior solution in the colloid in which both constituents 
are within the range of the molecular attraction of the mass.” 


. 1J. Chem. Soc., 105, 313 (1914). 
* Kollotdchem. Bethefte, 9, 1 (1918). 


5) 


4 THE HY DROUS OXIDES 


Wolfgang Ostwald! considers gels to be two-phase liquid-liquid 
systems possessing an interfacial tension. The vast majority 
of investigators, however, incline to the view that jellies are two- 
phase solid-liquid systems in which there is a network or cellular 
arrangement of solid phase permeated by liquid. 

The Solid-solution Theory.—The evidence in support of 
the solid-solution theory of jelly structure has been drawn largely 
from investigations on the swelling of substances. Thus in an 
exhaustive monograph, published in 1917, Katz points out the 
close similarity between the phenomena associated with swelling 
and the changes which accompany the formation of binary liquid 
mixtures. This parallelism would indicate that the swelling 
process is simply the formation of a solid solution between water 
and the swelling substance. Later, however, Katz? studied the 
effect of swelling on the x-ray spectrum of a number of substances 
to determine whether the taking up of liquid is intermicellar or 
intramolecular. If the liquid is held between the particles, the 
crystal lattice should not be altered by swelling, whereas if a 
solid solution is formed, the dimensions of the lattice should be 
increased. In practically all cases investigated, Katz observed 
no change in the x-ray spectrum, indicating that, as a rule, the 
swelling process is not a solid-solution phenomenon. 

From a study of the swelling of gelatin in acid solution, Procter 
concludes that gelatin combines with acid, forming easily soluble, 
highly-ionized salts, and that the volume of a swollen jelly under 
equilibrium conditions is determined by the osmotic pressure of 
the salts and the Donnan equilibrium. This view seems inade- 
quate to account for the marked increase in viscosity and the 
loss of mobility when a warm gelatin solution is cooled. In 
order to get around this difficulty, Procter postulates the forma- 
tion of tenuous and possibly flexible crystals which interlace and 
anastomose when a warm solution sets to a jelly on cooling. 
These crystals are assumed to be so very minute and the network 
so extremely fine that both solvent and crystals are within the 


1 Pfliiger’s Arch., 109, 277 (1905); 111, 581 (1906). ‘‘Theoretical and 
Applied Colloid Chemistry,” translated by Fischer, 103 (1917). 

2 Koninklijke Akad. Wetenschappen Amsterdam, 33, 281 (1924); Physik. Z., 
25, 321 (1924); Karz and Mark: Jbid., 33, 294 (1924); Chem. Zentr., I, 
442 (1924); Z. physik. Chem., 115, 385 (1925). 


JELLIES AND GELATINOUS PRECIPITATES 5 


range of each other’s molecular attraction. From these cond- 
siderations, it would appear that the only essential difference 
_ between the solid-solution theory and the two-phase solid-liquid 
theory is in the size of the particles constituting the network. 
Since these particles are not infrequently of microscopic dimen- 
sions, the solid-solution theory cannot be of general application. 

The Emulsion Theory.— Wolfgang Ostwald’s theory that jellies 
are simple emulsions of spherical or more or less distorted glob- 
ules in a liquid medium meets with serious objection at the - 
outset, since there are no emulsions known that have really the 
properties of jellies. The inorganic jellies certainly could not 
be looked upon as emulsions particularly in those cases where a 
rigid crystalline structure has been detected. Recalling the 
applicability of Boltzmann’s theory! which considers molecules 
to be completely elastic material particles incapable of much 
deformation, and van der Waals’ view” that the properties of 
molecules must be compared with those of solids, Zsigmondy? 
assumes, as seems necessary, that the larger ultramicrons of a 
solid are themselves solid. The liquid properties of gels rich in 
water are explained by assuming that the ultramicrons are 
surrounded by water layers and have a certain free path and 
motion. Hatschek* examined the emulsion hypothesis critically 
and found it untenable if the assumptions necessary to allow of 
mathematical treatment are granted. 

The Cellular or Honeycomb Theory.—The oldest theories of 
jelly structure were alike in picturing the bodies as two-phase 
solid-liquid systems; but there has long existed a fundamental 
difference of opinion as to the exact nature of the solid framework 
which is assumed to entrain the liquid phase and the manner in 
which this framework is formed. 

From an extended investigation first on foams and emulsions 
and later on gelatin, agar, and silicic acid jellies, Biitschli® 
concluded that the droplets of liquid were held in a cell-like 


1 “‘Vorlesungen tiber Gastheorie,’’ Leipsig, 34 (1896). 

2 “Die Kontinuitit des gasférmigen und flussigen Zustandes,”’ Leipsig, 34 
(1899). 

3 ““Chemistry of Colloids,” translated by Spear, 138 (1917). 

4 Trans. Faraday Soc., 12, 17 (1916). 

5 “Untersuchungen tiber Strukturen,”’ Leipsig (1898). 


6 THE HYDROUS OXIDES 


framework comparable to a honeycomb, an idea suggested, in 
all probability, by the cellular structure of the stems of young 
plants which enclose a relatively high percentage of water and 
still possess considerable rigidity. The walls of the cells in a 
silica jelly appeared to be about 0.3 » in diameter and the pockets 
which held the liquid from 1 to 1.54 in diameter. Gelatin jellies 
that appeared homogeneous under the microscope were hardened 
with alcohol or chromic acid to make their structure visible, and 
these likewise appeared to be made up of thin films. 

Bitschli’s general concept of jelly structure was supported by 
van Bemmelen,! Quincke,? and Hardy.* According to the latter, 
gelatin consists of two phases separated by a well-defined surface; 
one phase a solid solution of gelatin in water and the other a 
solution of water in gelatine. Like van Bemmelen, he assumes 
that both phases are liquid at first; but with fall of temperature, 
one becomes solid. The solid solution forms on the concave side 
of the surface of separation when the proportion of gelatin is 
small and on the convex side when the proportion of gelatin 
is large. In the latter case the drops of liquid are held in a solid 
gelatin-rich phase. As Bancroft* points out, such a jelly consists 
merely of a viscous medium in which liquid is dispersed and 
so does not have a honeycomb structure in the same sense that 
an emulsion has a honeycomb structure. The view entertained 
by Bancroft is that both phases in a gelatin jelly are colloidal 
rather than solid solutions. Since water peptizes gelatin under 
certain conditions, there is no reason why gelatin or a gelatin- 
rich phase should not peptize water. The separate phases will, 
therefore, in the nature of things, never be homogeneous. 

The investigations of Biitschli, van Bemmelen, and Hardy 
seemed so conclusive that a decade ago the honeycomb theory was 
generally looked upon as established.®> But later investigations 
of Zsigmondy and his pupils disclosed errors in the optical obser- 
vations of Biitschli and Hardy and showed the heterogeneity of 
jellies to be of an entirely different order of magnitude from that 


1Z. anorg. Chem., 18, 14 (1898). 

2 Drude’s Ann., 9, 793, 969 (1902); 10, 478, 673 (1903). 
3Z. physik. Chem., 33, 326 (1900). 

4“ Applied Colloid Chemistry,’ 241 (1921). 

5 Cf, FrEUNDLICHY ‘ Kapillarchemie,” 475 (1909). 


JELLIES AND GELATINOUS PRECIPITATES ‘s 


which the latter supposed. By applying the laws of capillarity 
to van Bemmelen’s! results on the hydration and dehydration 
of silica gel, Zsigmondy” estimated the diameter of the pores to 
be 0.5yu, that is, 200 or 300 times smaller than Biitschli observed. 
This was confirmed by Anderson* who showed that the pores 
vary in size, some being as small as 10uy in diameter. Working 
by the same method, Bachmann‘ found that gelatin jellies hard- 
ened by alcohol or chromic acid contained very much finer spaces 
than Biitschli supposed. Apparently the structure observed by 
Biitschli and Hardy were artifacts produced by the action of the 
hardening agents on the much finer structure already existing.® 

In the light of the work of Zsigmondy and his pupils, Lloyd® 
postulates a porous but continuous, solid cellular framework to 
enclose the liquid. The gelatin is assumed to exist in two chemi- 
cal states: gelatin, per se, and gelatin in the form of soluble salts. 
On cooling a solution containing isoelectric gelatin and gelatin 
salts in equilibrium with free electrolytes, the insoluble isoelectric 
gelatin is believed to precipitate not as crystals but in a state of 
suspended crystallization forming a solid framework which is 
kept extended by the osmotic pressure of the soluble gelatin 
salts in solution. In support of this hypothesis, isoelectric 
gelatin and water, in the absence of so-called gelatin salts in 
solution, were found to form an unstable clot that contracted 
and squeezed out liquid. It would seem, therefore, that an 
electrolyte must be present to form a stable gelatin jelly in 
accord with the view of Jordis.? J. Alexander® suggests that what 
Lloyd calls “‘suspended crystallization’? may be a manifestation 
of the protective or crystal-inhibiting action of a portion of the 
gelatin solution. This would account for the fact that a jelly 


1“T)ie Absorption,” 198 (1910). 

2Z. anorg. Chem., 71, 356 (1911). 

3Z. physik. Chem., 88, 191 (1914). 

4Z. anorg. Chem., 100, 1 (1917). 

5 Cf. Pauuti: ‘Der kolloidale Zustand und die Vorgiinge in der Lebendigen 
Substanz,”’ Braunschweig (1902); A. Fiscuer: “Fixerung, Farbung, und 
Bau des Protoplasms,’’ 312 (1899). 

6 Biochem. J., 14, 165 (1920), cf. THomson: J. Soc. Leather Trades’ 
Chem., 3, 299 (1919). 

7Z. Elektrochem., 8, 677 (1902). 

8 “Glue and Gelatin,” 71 (1923). 


8 THE HYDROUS OXIDES 


formed of isoelectric gelatin and water alone is apparently 
unstable in the sense that it contracts and squeezes out some 
of the water. But because of the slight inherent tendency of 
gelatin to crystallize, it is doubtful whether the alleged increase 
in stability of a jelly in the presence of a trace of electrolyte is 
due to inhibition of the crystallization of the gelatin phase. 
It seems more probable that the presence of an adsorbed ion 
may influence the nature and size! of the agglomerated particles 
and so may have an effect on the stability. If an electrolyte 
is necessary to form a stable jelly, the amount is apparently 
very slight indeed, since Field? prepared such a jelly from a very 
highly purified gelatin. Sheppard and Elliott* see no need of 
postulating the existence of osmotic pressure to keep the jelly 
extended, if the isoelectric gelatin forms a rigid solid framework. 

The Micellar Theory.—The investigations of Zsigmondy and 
Bachmann which disproved the observations of Bitschli and 
Hardy resulted in a resuscitation of the micellar theory of Frank- 
enheim‘ and Nageli.» According to this, the earliest theory of 
jelly structure, distensible bodies were assumed to consist of 
small anisotropic erystal-like molecular aggregates which retain 
their identity even when the substance goes into (colloidal) 
solution. The micelles, as Nageli called the molecular aggre- 
gates, take up water in such a manner that they are surrounded 
by a water layer, the thickness of which is determined by the 
relative intensity of the attraction of the micelles for water and 
for each other. Zsigmondy’s earliest investigations with the 
ultramicroscope led him to conclude with Nageli that the jelly 
structure is granular or flocculent; but later, Zsigmondy and 
Bachmann® observed a fibrilar structure in addition to the 
apparently grainy structure met with in diluted gels of gelatin, 
agar, and hydrous silica. ‘The fibrils or threads are quite sharply 
defined in soap jellies studied by Bachmann and later by McBain 


1WeisEer: J. Phys. Chem., 21, 314 (1917). 

2 J. Am. Chem. Soc., 43, 667 (1921). 

3 J. Am. Chem. Soc., 44, 373 (1922). 

4 “Tie Lehre von der Kohasion,’’ Breslau (1835). 

5 ““Pfanzenphysiologischen Untersuchungen,” Zurich (1858); ‘Theorie 
der Garung,’’ Munich (1879). 

6 Kolloid-Z., 11, 150 (1912). 


JELLIES AND GELATINOUS PRECIPITATES 9 


and his coworkers,' and in barium malonate jellies studied by 
Flade.* The latter noted the crystalline character of the fibrils 
and suggested that jellies in general probably consist of a net- 
work of crystalline threads.* Gortner* prepared a jelly of 
di-benzoyl-l-cystine which was found to consist of minute 
crystalline needle-like fibrils. Biichner®> showed that jellies, 
obtained from myricyl alcohol dissolved in chloroform and in 
amyl alcohol, consist of a conglomerate of very fine crystals 
which retain a large amount of liquid in the meshes. Bradford? 
champions the theory that the reversible sol-gel transformation 
is merely an extreme case of crystallization. Ultramicroscopic 
examination of a gelatin jelly reveals the presence of spherites 
which Bradford believes are made up of crystalline particles. 
Moeller’ likewise believes gelatinization to be a kind of crystal- 
lization in which there is formed a lattice of crystal threads that 
entrains the liquid; and von Weimarn® concludes from his inves- 
tigations that a jelly is a sponge composed of highly dispersed. 
crystalline granules soaked in dispersive medium. 

While Bradford, Moeller, and von Weimarn may have suffi- 
cient evidence to convince them of the crystalline character of all 
jellies, it is difficult for me to accept the view that there is no 
such thing as an amorphous precipitate of the flocculent, gelati- 
nous or jelly-like type. The theory that jelly formation is merely 
a process of crystallization seems to be contradicted by the 
work of Bogue, McBain, and Barratt, although all of the latter 
are strong supporters of a filamentousstructure. Bogue?® believes 
the elastic jellies such as gelatin to be made up of streptococcal 


1Lainc and McBain: J. Chem. Soc., 117, 1506 (1920); Darke, McBain, 
and Saumon: Proc. Roy. Soc. (London), 98A, 395 (1921). 

2Z. anorg. Chem., 82, 173 (1913). 

3 Cf. Sripei: Pfliiger’s Arch., 156, 361 (1914); HowEuu: Am. J. Physiol., 
40, 526 (1916). 

4 J. Am. Chem. Soc., 43, 2199 (1921). 

5 Rec. trav. chim., 42, 787 (1923). 

6 Cf. Fiscurr and Bosertaa: Jahresber. schles. Ges. vaterl. Kultur, 86, 33 
(1909). Chem. Zentr., I, 262 (1909). 

7 Biochem. J., 12, 351 (1918); 14, 91 (1920); 15, 553 (1921). 

8 Kolloid-Z., 23, 11 (1918). 

9 J. Russ. Phys.-Chem. Soc., 47, 2163 (1915). 

1 Chem. Met. Eng., 23, 61 (1920); J. Am. Chem. Soc., 44, 1343 (1922). 


10 THE HYDROUS OXIDES 


threads of molecules. According to his view, the catenary threads 
are very short and but slightly swollen in the sol condition, but 
elongate and absorb a great deal of water as the temperature falls 
and the sol starts to gel. A solid jelly results when the relative 
volume occupied by the swollen molecular threads is so great 
that freedom of motion is lost and the adjacent, heavily swollen 
aggregates cohere. 

Although it is possible for colloidal particles to possess the 
thread-like characteristics essential for forming an entangling 
mesh in which each particle is discrete, it seems more probable 
that in most cases the micelles actually become stuck together or 
orientated into loose aggregates which may take the form of 
chance granules, threads, or chains. Such a linking together of 
the particles to form an enmeshing network seems essential in 
some of the extremely dilute hydrous oxide jellies to which I 
shall refer later on. Laing and McBain! consider the gelatiniza- 
tion of soap to result from the linking up of colloidal particles 
to form a filamentous structure. ‘‘The colloidal particles in 
soap and gel are the same; but whereas in the former they are 
independent, in a fully formed gel they become linked up prob- 
ably to form a filamentous structure.” The formation of the soap 
curd is looked upon as a phenomenon analogous to crystal- 
lization that is distinct from the process of jelly formation.? 
The conception of micellar orientation in the process of gelation 
is supported by a number of observations mentioned by Laing 
and McBain, among which are the following: the identity in sol 
and gel of the electrical conductivity,? and the lowering of the 
vapor pressure; the intensifying of the molecular movement by 
heat which overcomes the forces holding the particles and 
causes melting of the gel; the transformation of certain jellies, 
such as nitrocotton into sol, by mechanical stirring which breaks 
down the orienting bonds between the particles;* the absence of 
Brownian movement in soap or gelatin jellies;> the dependence of 
the apparent viscosity of sols on their previous treatment and 


1 J. Chem. Soc., 117, 1506 (1920). 

2 Cf. Piper and GRINDLEY: Proc. Phys. Soc. (London), 35, 269; 36, 31 (1923). 
3 Cf. ARRHENIUS: Oefvers. Stockholm Akad., 6, 121 (1887). 

4 Cf. ALEXANDER: ‘‘Glue and Gelatin,” 75 (1923). 

5 BACHMANN: Z. anorg. Chem., 73, 125 (1912). 


JELLIES AND GELATINOUS PRECIPITATES 11 


history which influence the degree of orientation of their particles ;! 
the tendency of the jelly structure to shrink and exude liquid— 
Synerize—as a result of the component of attraction in the 
orienting force between the particles; and the frequent occurrence 
of supersaturation and hysteresis with regard to gelation. To 
these should be added the observation of Walpole? that the 
refractive index of a gelatin-water system is a linear function of 
the concentration, and when plotted against the temperature, no 
break occurs at the point of gelation; and the findings of Bogue? 
that the viscosity-plasticity change in the sol-gel transformation 
is gradual and regular. | 

Barratt* observed in fibrin jellies a non-crystalline fibrillary 
structure which formed an enmeshing network. When the 
jelly was first formed by gelatinization of a fibrinogen sol, no 
fibrils could be detected, but later they became visible in the 
ultramicroscope. This growth of particles in jellies has been 
observed frequently and in some cases is unquestionably due to 
growth of crystals, notably with barium malonate and some of 
the arsenate jellies® and with the dyes, benzopurpurine and chrys- 
ophenene;® but in other cases, it is the result of the agglomeration 
of amorphous particles. In accord with this view Scherrer’ 
showed that certain rigid jellies like hydrous silicon dioxide and 
hydrous stannic oxide showed well-defined crystalline inter- 
ference figures as well as the characteristics of amorphous bodies, 
whereas gelatin jellies showed no signs of a crystalline structure. 
Harrison® obtained spherical coagulation forms of starch which 
resembled Bradford’s spherites; but he does not regard them as 
crystalline. 

In the course of their investigations, Zsigmondy and Bach- 
mann observed ultramicroscopically the formation of gelatin, 


1Cf. HarscHEK: Kolloid-Z., 13, 881 (1913). 

2 Kolloid-Z., 18, 241 (1913). 

3 J. Am. Chem. Soc., 44, 1313 (1922). 

4 Biochem. J., 14, 189 (1920). 

5 Dersz: Kolloid-Z., 14, 139 (1914). 

6 Harrison: ‘‘The Physics and Chemistry of Colloids and Their Bearing 
on Industrial Questions,’ report of a general discussion held jointly by the 
Faraday Society and Physical Societies of London, Oct. 25, 57 (1920). 

7 Nachr. Kgl. Ges. Wiss. Gottingen, 96 (1918). 

8 J. Soc. Dyers Colourists, 32, 40 (1916). 


12 THE HYDROUS OXIDES 


agar, and silica jellies by agglomeration into flaky groups of 
freely movable ultramicrons of unknown structure. It is thus 
implied that all jellies are not necessarily filamentous in structure. 
This is supported by recent ultramicroscopic observations carried 
out by Harrison! on gelatin and cellulose jellies which were found 
to consist of minute portions joined together in a somewhat irreg- 
ular manner. Alexander? believes that the formation of chains 
or threads is not essential to gelation, although chain-like struc- 
tures may form as a result of orientation of the polar molecules. 

Whatever may be the exact structure of jellies, most of the 
experimental evidence supports the micellar or sponge theory 
rather than the cellular or honeycomb theory. The presence of 
definite threads or filaments leaves little room to doubt the exist- 
ence of an interlacing network structure in certain jellies. It 
would, of course, be highly interesting, if jellies of widely different 
substances were all essentially identical in structure. Such a 
condition seems altogether unlikely; but investigators have 
apparently sought to establish such an identity. Studies on 
specific jellies have led some to conclude that all jellies are made 
up of a framework of amorphous threads; other that they are 
composed of crystalline threads; and still others who fail to find 
any threads or filaments at all but observe an irregular grouping 
of particles. Doubtless all are right in specific cases. Indeed, 
it is not unlikely that there are various arrangements of molecular 
aggregates in different jellies and perhaps in the same jelly. 
In a heterogeneous mixture of complex groups such as are found 
in gelatin sol or jelly, it is probable that the process of gelation 
and the jelly structure are more complex than in the inorganic 
jellies or in soap jellies. The orientation of the particles may 
result in fibrils in certain cases and in more or less irregular 
arrangements in others. In certain cases the fibrils may consist 
of definite crystals, while in others the crystalline characteristics 
may be entirely lacking. In all cases it seems probable that the 
particles are highly hydrous as a result of adsorption or absorp- 
tion and that they are linked together, forming an irregular mesh 
or network in the interstices of which liquid is entrained. 


1“ The Physics and Chemistry of Colloids and Their Bearing on Industrial 
Problems,” 57 (1920). 
2 “Glue and Gelatin,” 84 (1923). 


JELLIES AND GELATINOUS PRECIPITATES 13 


We may next inquire into the structure of gelatinous precipi- 
tates. Since a gelatinous precipitate differs from a jelly only in 
having undergone contraction with the consequent excretion of 
liquid, the two types of gels are generally considered to be quite 
similar in structure. Recent investigations of the physical 
character of bodies by means of x-rays confirm von Weimarn’s 
contention that many gelatinous precipitates, such as hydrous 
alumina and ferric oxide, which we used to think were amorphous, 
are, in reality, made of myriads of tiny crystals. This naturally 
raises the question whether the submicroscopic crystals are 
themselves gelatinous and so impart the gelatinous property to 
the mass. Unfortunately, von Weimarn does not enlighten us 
on this point; but it is apparently possible to have gelatinous 
crystals. Thus Harrison! speaks of aqueous solutions of benzo- 
purpurine and chrysophenene setting to jellies containing gelat- 
inous crystals, some of them so fine that they can pass unbroken 
through a filter paper. Similarly, cholic acid gives a blue precipi- 
tate with iodine which may form in clusters of needle crystals 
possessing rigidity. Under other conditions needle-shaped 
crystals are formed which are gelatinous and can be bent in all 
kinds of shapes by moving the cover glass on the microscope slide. 
Some of these so-called gelatinous crystals show remarkable 
vibrations due to the impact of the molecules and move about 
like the spiral bacteria present on the teeth. Harrison’s observa- 
tions seem to throw some light on the problem of what constitutes 
a gelatinous crystal or aggregate and hence on therelated problem 
of what is a gelatinous precipitate. 

Le Chatelier? succeeded in polishing metal with colloidal silicic 
acid and hence concluded that the gelatinous precipitate consists 
of anhydrous silica and water. Bancroft*® considers this evidence 
inconclusive since anhydrous silica may have been formed as a 
result of pressure during polishing, and suggests that a better 
method of attack is to consider whether grains of sand mixed 
with water will give a gelatinous precipitate. Since this does 
not happen, as a rule, Bancroft concludes: 


1“The Physics and Chemistry of Colloids and their Bearing on Industrial 
Problems,’’ 58 (1920). 

2 “Tia Silice et les Silicates,”’ 76 (1914). 

3“ Applied Colloid Chemistry,” 236 (1921). 


14 THE HYDROUS OXIDES 


We must therefore assume one of two things. Either the sand 
grains are held together extraordinarily firmly by water when they are 
very fine, or there is some other factor comes in. ‘The first explanation 
cannot be the right one because, if it were, one ought then to be able 
to get a gelatinous precipitate of any colloid at ordinary temperatures 
without much difficulty, which is not the case. We never get gelat- 
inous gold, and while we can get gelatinous calcium carbonate, we 
have to do it in a very special way. Consequently, Le Chatelier’s 
hypothesis cannot be accepted without modification. ; 


As previously noted, Zsigmondy! explains the liquid character 
of gels rich in water by assuming the ultramicrons to be sur- 
rounded by water layers and to have a certain free path and 
motion. The objection to this view is that Zsigmondy does not 
show why it should be so. Harrison’s observations on gelati- 
nous crystals bear on this point. Gelatinous crystals are appar- 
ently extremely fine, needle-shaped masses so thin that they lack 
rigidity and so flexible that they can be bent and twisted into 
various shapes and may move under the bombardment of water 
molecules. A cluster or network of such needle-shaped, flexible 
crystals that adsorb water strongly would form a viscous or 
plastic mass, usually known as a gelatinous precipitate. If the 
crystals are compact and rigid rather than thin and flexible, they 
would not form a gelatinous precipitate unless they united into 
threads or strings possessing the flexibility and elasticity which 
characterizes a thin needle crystal. Obviously the particles 
need not be crystalline, and as a rule they probably are not. A 
gelatinous precipitate 1s apparently a network composed of 
extremely finely divided particles which have coalesced to form 
flexible filaments or chains and which adsorb water very strongly 
and so are highly hydrous. Where the particles do not adsorb 
water particularly strongly and where the tendency to coalesce 
into filaments or threads is not great, a high concentration of the 
finely divided particles is necessary, as in the case of calcium car- 
bonate and barium sulfate. It is probable that neither tendency 
is very marked in the case of gold, which accounts for the fact 
that no one has prepared a gold jelly. I am not aware, however, 
that anyone has attempted to precipitate a fairly large amount 


1 ZstigMonpy: ‘‘Chemistry of Colloids,” translated by Spear 138 (1917). 


JELLIES AND GELATINOUS PRECIPITATES 15 


of gold in a small volume, as von Weimarn does with barium sul- 
fate. While a gelatinous precipitate of gold has not yet been 
prepared, this might be a fairly simple process if the metal were 
dispersed in some liquid, other than water, which is very strongly 
adsorbed by gold. Bdérjeson! working in Svedberg’s laboratory, 
prepared a cadmium jelly by allowing a very dilute sol of cad- 
mium in alcohol to stand for some time in a glass bottle. In 
this case the particles were only 5upy in radius and the concen- 
tration but 0.2 to 0.5 per cent. Barium sulfate is readily 
obtained in a gelatinous form by precipitation in selenium oxy- 
-chloride.2 The physical character of the precipitate is due to 
very strong adsorption of selenium oxychloride by the minute 
particles which form as a result of the extreme insolubility of the 
sulfate in the liquid medium. 


PREPARATION 


If we start out with the assumption that a gel consists of myri- 
ads of particles enmeshed into a network which entrains liquid, 
it follows that any substance should form a gel, provided a suit- 
able amount of a highly dispersed substance is precipitated and 
provided the particles adsorb the dispersing medium very 
strongly. The amount of the dispersed phase that must be 
present to form a firm jelly by a precipitation method will depend 
on the size and nature of the orientation of the particles and 
the extent to which they adsorb the dispersing liquid. The 
methods of procedure which have been employed will be con- 
sidered separately. 

Cooling of Sol.—Certain substances such as gelatin and agar- 
agar swell in water at ordinary temperatures but are not pep- 
tized, forming a sol, until the temperature is raised. At the 
higher temperature, the liquid phase serves the double role of 
peptizing agent and dispersing medium. On cooling such a 
sol, a jelly is formed provided the concentration is suitable. 


1“The Physics and Chemistry of Colloids and their Bearing on Industrial 
Problems,” 55 (1920). 
2 LENHER and Tayuor: J. Phys. Chem., 28, 962 (1924). 


16 THE HYDROUS OXIDES 


Thus a sol containing 1 per cent of pure gelatin does not gel 
until around 10°, and gelation does not take place at any concen- 
tration above +35°. According to Bachmann,! pure warm 
solutions of gelatin are almost homogeneous, but on cooling, a 
new phase appears, as evidenced by a heterogeneity that is 
amicroscopic or submicroscopic, depending on the concentration. 
This process is similar in certain respects to crystallization but 
differs from it in that microns, submicrons, ultramicrons, and 
amicrons are formed according to the concentration. The 
appearance of visible particles is not dependent on the formation 
of a jelly, as these may be seen before the jelly sets and in dilute 
solutions that do not set. When a jelly results on cooling a sol, 
the process apparently consists in the formation of highly 
hydrous molecular aggregates which are linked together to 
form a more or less rigid network. Bogue believes that the 
aggregates not only grow but become more hydrous on cooling. 
This might’ be expected in view of the rapid increase in adsorp- 
tion which usually results from lowering the temperature. The 
sol-gel transformation in a given system does not occur at a 
definite transition point, but the transition is continuous and 
reversible over a somewhat indefinite period.? 

Swelling. Non-aqueous Gels.—Practically all substances 
which form the so-called elastic gels show the capacity of swelling 
in a suitable liquid. Thus dry gelatin, fibrin, and starch will 
swell in water at ordinary temperature, forming jellies that are 
peptized at higher temperatures giving sols. Similarly, albumin 
swells in water but not in alcohol, benzene, ether, or 
turpentine. Vulcanized india rubber swells in various organic 
solvents such as benzene, toluene, and xylene but not in water; 
and soaps swell in water and in many organic solvents. Numer- 
ous theories? have been advanced to explain the phenomenon, 
but there is as yet no explanation to account for the fact that 
certain substances swell in only a limited number of liquids. 
The swelling of gelatin has been studied most extensively and 
has been found to depend on a number of factors, among which 


1Z. anorg. Chem., 73, 125 (1911). 

2 Bocur: J. Am. Chem. Soc., 44, 13813 (1922). 

3'These theories have been summarized and their limitations pointed out 
in a paper by BarTEuy and Sims: J. Am. Chem. Soc., 44, 289 (1922). 


JELLIES AND GELATINOUS PRECIPITATES ee 


may be mentioned the hydrogen ion concentration;! the addition 
of neutral. salts;? the temperature; and the structure.* 
The importance of the hydrogen ion concentration on the swell- 
ing phenomenon was suggested by Ostwald and has been empha- 
sized particularly by Procter and Wilson and by Loeb, who have 
applied Donnan’s theory of membrane equilibria in interpreting 
the mechanism of the swelling process. Before taking up the 
Procter-Wilson theory of swelling, the theory of membrane 
equilibria on which the former is based will be considered briefly. 
Donnan’s theory of membrane equilibria’ deals with the equilib- 
ria resulting when a membrane separates two electrolytes 
containing 1 ion which cannot diffuse through the membrane. 
Starting with two completely ionized electrolytes, (1) NaCl and 
(2) NaR, separated by a membrane impermeable to the ion 
R’, Donnan shows that equilibrium will be established only 
when the product of the concentration of sodium and chloride 
ions has the same value on both sides of the membranes, thus, 


[Na’]i X[Cl’], = [Na’}e X[CI’]2 


the brackets signifying concentration in mols per liter, and the 
subscripts 1 and 2 referring to solution 1 and solution 2, respee- 
tively. This is the so-called equation of products based on the 
distribution law. 

For the specific case cited above, the equation of products 
may take a somewhat different form. Thus, at the outset, the 
system of two solutions separated by a membrane may be repre- 
sented as follows: 


‘Solution 1 Solution 2 
[Na’ ][Cl’] [Na ][R’] 


1 CurtAri: Biochem. Z., 38, 167 (1911); Procrmr: J. Chem. Soc., 106, 313 
(1914); Lozs: J. Gen. Physiol., 1, 41 (1918). 

2 HorMeiIsTer: Arch. exptl. Path. Pharmakol., 27, 395 (1890); 28, 210 
(1891); Pau: Pfliiger’s Arch., 67, 219 (1897); 71, 333 (1898); Sprro: 
Beitrége zur chem. Physiol., 5, 276 (1904); Wotreane OstwaLp: PYltiger’s 
Arch., 108, 563 (1905); Fiscnmr: ‘‘Hdema,” New York (1910). 

3ProcteR and Burton: J. Soc. Chem. Ind., 35, 404 (1916); Arisz: 
Kolloidchem. Bethefte, T, 42 (1915). 

4Z. Elektrochem., 17, 572 (1911). 


18 THE HYDROUS OXIDES 


On allowing the system to stand, the diffusible sodium and chlo- 
ride ions distribute themselves until equilibrium is established. 
At equilibrium in solution 1, let 


a teso| NG ue Ce 


and at equilibrium in solution 2 let 


y = [Cl] 
and a =| hye 
hence, | (y +z) = [Na’] 
Thus we have 
Solution 1 Solution 2 
— £Na-* Lov le + 2)Na:* Yor’ 2p 


and the equation of products is 
P= ty are 2) 


In solution 1, zy,- = “cv, while in solution 2, yng: + 2na: = Yer 
but since the product of the concentrations in solution 1 is the 
same as the product of the concentrations in solution 2, it must 
follow that 

Na + Lov < YNa + YCv + 2na- 
or 22 <2y + 2 


In other words, at equilibrium, the concentration of diffusible 
ions in solution 2 is greater than in solution 1. Now, if we let 


e= (2y+ 2) — 22 
then 2yte=e+ 2x 


and the equation of products becomes 
e=y t+ Vey 


which shows again that sodium chloride does not distribute itself 
equally, but the concentration of the ionized sodium chloride at 
equilibrium is greater in solution 1 than in solution 2. This gives 
rise to an osmotic-pressure difference as well as to a difference in — 
potential across the membrane. The equation for this potential 
difference was derived by Donnan in the following way: 

Let 7, and ze be the potential for positive electricity in solu- 
tion 1 and solution 2, respectively, in the above mentioned system; 


JELLIES AND GELATINOUS PRECIPITATES 19 


_ and let the minute amount of positive electricity Fdn be trans- 
ferred isothermally from solution 2 to solution 1. This process 
involves a change in free electrical energy represented by Fdn 
(7; — m2) and the simultaneous transfer of udn mols of Na’ from 
solution 2 to solution 1 and of vdn mols of Cl’ from solution 1 to 
solution 2, where u and v are the transport numbers of the respec- 
tive ions. The maximum osmotic work involved in the transfer 
of the ions is given by the expression 


[Na ]2 
[Na]: 


[Cl]: 
[Cl’]e 








udn RT log. + vdn RT log. 


Since the system is in equilibrium, the electrical work is equivalent 
to the osmotic work, or 











Fdn (a1 — m2) = udn RT loge tN : +vdn RT log. are 
2 
praises ACM): « at 
Bu MN: an = cl; =jandu +1 = 
Hence, if Cs Ty <— 19 
RT 
= - log | 


It may be shown that this equation is valid even when other 
ions of any valence are added to the system. Donnan has tested 
- the accuracy of the equation in the following cases: (1) Congo 
red and sodium chloride; (2) potassium chloride and potassium 
ferricyanide; (3) sodium arsenate and sodium chloride. In 
every instance there is fairly good agreement between the calcu- 
lated and experimental values. Donnan has also applied the 
same general principles to such cases as NaA and KA, and NaA 
and CaAz, in which the membrane is impermeable to the ion A’. 

The Procter-Wilson Theory of Swelling..—To account for the 
swelling of gelatin, Procter and Wilson assume that hydrochloric 
acid, say, combines with gelatin forming a readily soluble high- 
ionized salt, gelatin chloride, and that the resulting equilibrium is 
a special case of the Donnan membrane equilibria. To make the 


1 Procter: J. Chem. Soc., 105, 313 (1914); Kolloidchem. Bethefte, 2, 243 
(1911); Proctrr and Wiuson: J. Chem. Soc., 109, 307 (1916); Winson and 
Witson: J. Am. Chem. Soc., 40, 886 (1918). 


20 THE HYDROUS OXIDES 


reasoning general, a protein G is supposed to react with an acid 
HA in accord with the following equation: 


G+ H’ + A’ = GH’+ A’ 


Hence, if a millimol, say, of G is immersed in a solution of HA, 
the solution penetrates the jelly which combines with a part of 
the H‘ions giving GH’. In this way the concentration of H’ 
ions within the jelly is reduced below that of the A’ ion; whereas 
in the solution surrounding the jelly, the concentrations of the 
two ions are necessarily equal. ‘Thus the solution is separated 
into two phases, an external phase with the two diffusible ions 
H’° and A’ and a jelly phase containing the diffusible ion A’ and 
the ion GH" which is a part of the elastic jelly structure and so 
cannot diffuse. This constraint imposes a restraint on the equal 
distribution of ions within and without the jelly. When equilib- 
rium is established, in the external phase, let 


x = (H' | ={Aq 
and in the jelly let yl Hy 
and PA =map eet 
From which [A’]=yt+e 


Since the product [H’] x [A’] will have the same value in both 
phases at equilibrium, it follows that 


x? = y(y + 2) 
If, as before, we let e= 2y +2— 2x 
then x=yt /ey 


This shows z to be greater than y, which means that [H ‘]is greater — 
outside the jelly than in it. From this it follows that [A’] is 
greater in the jelly than in the external solution. For this reason 
the anions of the protein salt will tend to diffuse outward into 
the external phase. This exerts a pull on the cations GH° 
forming part of the protein framework, and causes an increase in 
the volume of the jelly directly proportional to e, the excess of 
concentration of diffusible ions of the jelly over that of the 
external solution. 

Procter and Wilson have tested this theory experimentally 
in the case of gelatin and hydrochloric acid, and have found good 


JELLIES AND GELATINOUS PRECIPITATES 21 


agreement between observed and calculated values. Moreover, 
Loeb and Kunitz! showed that all monobasic acids produce 
approximately the same degree of swelling at the same hydrogen 
ion concentration, as the theory predicts. 

The addition to acid-swollen jellies of neutral salts, such as MA, 
neither of whose ions combine with the protein as hydrogen ion 
is supposed to do, increases the cation concentration y in the 
jelly but not the alleged gelatin cation concentration z. This 
decreases the excess of diffusible ions inside the jelly over that 
outside and so decreases the swelling, as observed experimentally. 

The application of the Donnan theory of membrane equilibria 
to the swelling of gelatin is certainly a step forward in explaining 
the mechanism of the swelling process, although it is apparently 
inapplicable to such cases as the swelling of rubber in benzene 
or xylene, where the existence of a Donnan equilibrium is pre- 
cluded by the absence of dissociation. Moreover, it is a serious 
mistake to conclude, as some have done, that prediction of results 
by means of a formula proves the assumptions on which the for- 
mula is based. Procter, Wilson, and Loeb assume a definite 
chemical combination between gelatin and hydrochloric acid 
with the formation of a highly ionized salt, gelatin chloride, which 
gives a non-diffusible cation GH’. The mathematical formulas 
deduced from this hypothesis do not prove its correctness, for 
one can get exactly the same formulas and make exactly the 
same predictions by making the more probable assumption that 
hydrogen ion is preferentially adsorbed on the surface of gelatin 
particles rather than entering into definite chemical combination 
with the particles. This is recognized clearly by Donnan:? 


Very many interesting investigations based on this simple theory 
have been made by Jacques Loeb and his collaborators. In this work, 
among other things, the effects of acids, alkalies, and salts on the osmotic 
pressures and membrane potentials of the amphoteric proteins have 
been studied. Loeb has shown that the simple theory of membrane 
equilibria is capable of accounting fairly quantitatively for a great 
many of his experimental results, and regards this as a proof that the 
phenomena exhibited by the protein ampholytes are due to simple 
chemical reactions and not to the adsorption of ions by colloid aggre- 


1 J. Gen. Physiol., 5, 665, 693 (1923). 
2 Chem. Reviews, 1, 87 (1924). 


22 THE HYDROUS OXIDES 


gates or micelles. While this view may be correct in many instances, 
it is necessary to remember that the theory of membrane equilibria 
depends simply on two assumptions: (a) the existence of equilibrium; 
(b) the existence of certain constraints which restrict the free diffusion 
of one or more electrically charged or ionized constituents; and that 
the equations which result from the theory will hold equally well 
whether we have to deal with ‘‘colloid units’? which have acquired an 
ionic character (electrical charge) by adsorption of ions, or with simple 
molecules which have become ionized by the loss or gain of electrons. 
All that is necessary for the theory is that the simple ionized molecules 
of the ionic micelles be subjected to the same constraint, namely, inabil- 
ity to diffuse freely through the membrane. This constraint then 
imposes a restraint on the equal distribution on both sides of the mem- 
brane of otherwise freely diffusible ions, thus giving rise to the concen- 
tration, osmotic, and electrical effects with which the theory deals. 


The investigations of Loeb led him to conclude that only the 
anions of neutral salts are taken up by gelatin on the acid side 
of the so-called isoelectric point of gelatin (pH = 4.6) and only 
cations on the alkaline side. This conclusion is hardly justified 
by Loeb’s experiments since, throughout most of the range 
investigated, he was working with relatively low concentrations 
of salts and so detected no effect of cations other than hydrogen 
on the acid side and of anions other than hydroxyl on the basic 
side. At relatively high concentrations of neutral salts, the 
specific effect of cations other than hydrogen and of anions 
other than hydroxyl would doubtless appear. This inference 
is supported by work carried out in the author’s laboratory on 
the adsorption of anions by hydrous chromic oxide on the alkaline 
side of the isoelectric point. If the concentration of the anion 
under consideration is very large relatively to that of hydroxyl, 
the effect of the latter is negligible, whereas if the hydroxyl ion 
concentration is appreciable, the adsorption of the other ion is 
cut down enormously or completely nullified.! 

As noted above, the dehydration and swelling of a gelatin 
jelly is reversible over a considerable range. ‘This is not the case 
with hydrous oxide jellies such as silica. Van Bemmelen? 


1Cf. MicHaEuis: Colloid Symposium Monograph, 2, 1 (1924); Strasny: 
Kolloid-Z., 35, 353 (1924). 
2 “Die Absorption” (1910). 


JELLIES AND GELATINOUS PRECIPITATES 23 


showed that a silica gel containing a great deal of water shrinks 
very much when the water is removed; and, while it will take up 
some water again, the volume change is not reversible. If the 
drying is carried sufficiently far, pores are developed that are 
filled with air, and these pores can then be filled with a liquid 
other than water; but there is no appreciable swelling. When 
gelatin is dried, such pores are not developed and a dry gel of 
natural gelatin will not adsorb benzene. 

Although the porous mass formed by drying a non-elastic 
gel will not swell in organic liquids, Graham found that such 
liquids will replace the water in a jelly. Thus a silica gel con- 
taining 11 per cent SiO». was suspended repeatedly in alcohol, and 
an alcogel was formed having approximately the same volume 
as the original gel. In a similar way the water was replaced by 
inorganic and organic acids. Van Bemmelen substituted ace- 
tone for the water and Bachmann! put in benzene. Neuhausen 
and Patrick? found that the replacement of water was not quite 
so complete as Graham reported on repeated immersions of a 
silica jelly in anhydrous alcohol or benzol. Elastic jellies show a 
similar behavior. Thus Biitschli? found it comparatively easy 
to replace the water in a gelatin jelly with alcohol and this again 
by chloroform, turpentine, or xylene, even popes dry gelatin 

does not swell in these liquids. 
Concentrated Gels.—Many difficultly soluble salts that ordi- 
narily precipitate in relatively large crystals can be thrown out in 
the form of a gelatinous precipitate or jelly from very concen- 
trated solutions. This phenomenon was observed by Hartung,’ 
Biichner,® Biedermann,® Neuberg,’ and particularly by von Wei- 
marn.® The latter? made a systematic study of the form in which 
substances precipitate from solution. He calls attention to a 


1Z. anorg. Chem., 78, 125 (1912). 

2 J. Am. Chem. Soc., 48, 1844 (1921). 

3 “Uber den Bau quellbarer Korper,” Gottingen, 22 (1896). | 

4 “Recherches de morphologic synthétique sur la production artificielle de 
quelques formations calcaries organiques,’’ Amsterdam (1872). 

5 Chem. Ztg., 17, 878 (1893). 

6 Z. allgem. Physiol., 1, 154 (1902). 

7 Siteb. Akad. Wiss. Berlin, 820 (1907). 

8 “Zur Lehre von den Zustiinden der Materie’”’ (1914). 

9 Von WermARN: ‘‘Grundziige der Dispersoidchemie,” 39 (1911). 


24 THE HYDROUS OXIDES 


number of very different factors on which precipitation depends: 
the solubility of the substance; the latent heat of precipitation; 
the concentration at which the precipitation takes place; the 
normal pressure at the surface of the solvent; and the molecular 
weights of the solvent and the solute. He points out the impossi- 
bility of taking all of these factors into account and simplifies 
the problem by considering, first, but two of the factors: the 
solubility of the precipitating substances, and the concentration 
at which precipitation begins. The effect of viscosity is discussed 
briefly in a later work.! The process of condensation (precipita- 
tion) is divided into two parts: the first stage, in which the mole- 
cules condense to invisible or ultramicroscopic crystals; and the 
second, which is concerned with the growth of the particles as a 
result of diffusion. The velocity at the important first moment 
of the first stage of the precipitation is formulated thus: 


Condensation pressure es neat Ee KE ae a 


W=K Condensation resistance L L 





where W is the initial rate of precipitation; Q the total concen- 
tration of the substance that is to precipitate; L the solubility of 
coarse crystals of the substance; Q — L = P the amount of super- 


rele ; 
saturation. The ratio jcoe U is the precentage supersatura- 


tion at the moment precipitation begins. 

The velocity of the second stage is given by the Nernst-Noyes 
equation: 

Ve me O-(C —1D) 
as 

where D is the diffusion coefficient; S the thickness of the adherent 
film; O the surface; C the concentration of the surrounding solu- 
tion; and / the solubility of the dispersed phase for a given degree 
of dispersity. C — 1 may be termed the absolute supersaturation. 

From these general formulations, von Weimarn arrives at the 


conclusion that jellies are obtained only when the ratio ee that 


is, the percentage supersaturation U, can be made enormous. 
It is pointed out that the nature of a precipitate is quite different, 


1Von WEIMARN: Kolloidchem. Bethefte, 4, 101 (1912). - 


JELLIES AND GELATINOUS PRECIPITATES 25 


depending on whether a given value of U is obtained by a large 
Por byasmall Zl. If a large U is obtained by a high value of 
P, a large amount of disperse phase is produced and a gel forms, 
while if P is small and L very small, a relatively small amount of 
disperse phase is produced and a sol is formed. Von Weimarn 
has demonstrated the accuracy of his deductions in a large num- 
ber of cases, using reacting solutions of high concentrations; and 
it is apparently true that any salt can be obtained in a gelatinous 
form if the concentration of the reacting solutions and so the 
velocity of precipitation is sufficiently high. Thus, vonWeimarn! 
prepared gelatinous precipitates of barium sulfate which usually 
comes down in the form of crystals, by mixing 1 to 3 N solutions 
of manganese sulfate and barium thiocyanate. By using solu- 
tions of sufficiently high concentration (3 to 7N) all the solute 
was enclosed, forming jellies. These are not the conditions under 
which jellies are usually obtained, and their existence is temporary. 
By mixing very high concentrations of materials that react to 
form an insoluble precipitate, a very large number of relatively 
small particles are formed, because of the high degree of super- 
saturation.2 Each of these minute particles adsorbs a little water 
and as they are very close together, a semisolid mass results that 
entrains all the liquid phase, thus forming what has been termed 
a jelly. These so-called jellies break down on standing, on 
account of growth of the particles and the consequent liberation of 
adsorbed water. I do not believe that precipitates in which the 
ratio of mols of water to mols of salt is, say, 20:1 or 25:1 should 
be considered as jellies in the same sense as precipitates in which 
this ratio is two or three hundred times as great. Very finely 
divided sand or fuller’s earth may be matted in the bottom of a 
test tube, and this solid will take up a great deal of water before a 
supernatant water layer is observed; but I should not call such 
a preparation a jelly. It seems to me that von Weimarn’s barium 
sulfate jelly may be similar except that the particles are much 
smaller, and so a given amount will take up more water. On 
the other hand, with true jellies where the amount of enclosed 
water may be relatively enormous, time must be allowed for 


1 ‘Zur Lehre von den Zustiinden der Materie,” 21 (1914). 
2 BancrorT: J. Phys, Chem., 24, 100 (1920). 


26 THE HYDROUS OXIDES 


formation of a definite structure. As a matter of fact, von 
Weimarn! recognized a difference between a barium sulfate jelly 
. prepared by his method and a jelly formed by uniform gelatiniza- 
tion of a liquid throughout its mass, as in the case of gelatin 
jelly. The former he terms a ‘‘coarsely cellular gel’? and the 
latter a ‘“‘reticulated gel.” | 

Precipitation of Sol.—Since finely divided particles that adsorb — 
water strongly are of primary importance for the formation of a 
hydrous jelly, it would seem that the most promising method of 
preparing dilute jellies would be to precipitate hydrous substances 
from colloidal solution. The von Weimarn theory would tell 
us, of course, that this precipitation would have to take place 
at a suitable rate under conditions that are not conducive 
to growth of the individual particles; but it does not enable us 
to predict the optimum rate of coagulation, the effect of salts on 
jelly formation, or the conditions that favor the formation of a 
jelly rather than a gelatinous precipitate. As a result of recent 
investigations in the author’s laboratory on the formation of 
typical dilute inorganic jellies, the hydrous oxides particularly, 
it is possible to outline the general conditions of jelly formation 
and the effect on the process of various factors other than the 
percentage supersaturation ‘‘at the important first moment of 
the first stage of condensation” from molecules to invisible 
particles. Jellies would be expected to form from colloidal 
solution if a suitable amount is precipitated at a suitable rate 
-without agitation in the absence of a medium that exerts an 
appreciable solvent or peptizing action. If the concentration 
of the colloid is too low, no jelly or only a very soft jelly can result. 
If the velocity of precipitation is too great, contraction is likely 
to occur with the formation of a gelatinous precipitate instead of a 
jelly. The effect of the presence of salts on jelly formation is, 
therefore, determined in large measure by the precipitating and 
stabilizing action of the ions in so far as these affect the rate of 
precipitation. In general, a slow rate of precipitation favors the 
formation of a jelly rather than a gelatinous precipitate if there 
is little or no tendency of the particles to grow as a result of the 
solvent action of the electrolyte. The favorable concentration 
for different electrolytes is in the immediate region of their 

1 J, Russ. Phys.-Chem. Soc., 47, 2163 (1915). 


JELLIES AND GELATINOUS PRECIPITATES 27 


precipitation concentration. A little below this value, no precipi- 
tation or only a slight precipitation takes place; while above this 
value, coagulation is usually so rapid that a gelatinous precipitate , 
is formed instead of a jelly. The reason is that time is not allowed 
for the uniform mixing of the colloid with coagulant, and the 
slow uniform precipitation necessary for the building of a uniform 
jelly structure is replaced by rapid uneven coagulation and the 
consequent contraction that distinguishes a gelatinous precipitate 
from a jelly. 

The accuracy of these deductions has been demonstrated 
repeatedly, and frequent reference will be made to them in later 
chapters. In many cases, these jellies may be obtained in rela- 
tively low concentrations. A notable example is the case of 
hydrous chromic oxide which formed a firm jelly containing but 
0.18 per cent Cr2O; and a soft jelly containing 0.09 per cent Cr2QO3. 
The formation of such dilute jellies can result only when the 
particles are very hydrous and when the conditions of precipitation 
allow time for the building up of an enmeshing network. In 
ease the particles are but slightly hydrous and show but little 
tendency to link together into threads, extremely high concen- 
trations must be present, as von Weimarn found. 

Dialysis of Sols.—Prolonged dialysis of colloidal solutions 
frequently leads to the precipitation of a part of the suspended 
phase as a gelatinous precipitate. When this process was carried 
out in a suitable way on a colloidal solution of ferric arsenate 
peptized by ferric chloride, Grimaux! obtained a firm, transparent 
jelly. This observation has been confirmed and extended by 
Holmes and his pupils. Similar observations have been made 
in the author’s laboratory with hydrous oxides of chromium and 
aluminum, and the method is probably a general one. From 
the point of view outlined in the foregoing section, the formation 
of jellies by dialysis of a colloidal hydrous substance is readily 
understood. Dialysis merely removes the stabilizing ion slowly 
and uniformly below the critical value necessary for peptization; 
and precipitation results just as if the adsorption of the stabilizing 


1 Compt. rend., 98, 1540 (1884). 

2 Hotmes and Rinprusz: J. Am. Chem. Soc., 38, 1970 (1916); Hotmes 
and Arnoup: Jbid., 40, 1014 (1918); Hotmes and Fatt: Jbid., 41, 763 
(1919). 


28 THE HYDROUS OXIDES , 


ion were compensated for or neutralized by the addition of an 
electrolyte having a suitable precipitating ion. The accuracy of 
these deductions has been demonstrated conclusively in a series 
of investigations on the arsenates of iron and aluminum.! 

Dilute Jellies by Metathesis.—According to the von Weimarn 
theory, mixing dilute solutions that interact at once may give 
a gelatinous precipitate but. not a jelly, since the percentage 


supersaturation = U is too small because of the small value of 


P. As a matter of fact, however, jellies have been obtained 
under certain conditions by mixing very dilute solutions in which 
L is sufficiently large that precipitation is slow and quantitative 


precipitation impossible, so that - = U is very small. Such 


cases are apparently not covered by the von Weimarn theory. 
It is quite possible to obtain a gelatinous precipitate by mixing 
dilute solutions of two salts which precipitate immediately 
(P small, but Z very small); but a jelly will not form under 
these conditions. The reason is evident when we consider the 
impossibility of getting the instantaneous mixing of the solutions 
which is essential for uniform precipitation throughout the 
mixture. One part is precipitated before another is mixed with 
the precipitant, and the uniformity characteristic of a jelly 
is lost. Moreover, the mixing itself will tend to destroy the 
jelly structure. The results are, therefore, not unlike those 
obtained when a colloid capable of forming a jelly by slow precip- 
itation is coagulated too rapidly by the addition of excess elec- 
trolyte. To obtain a jelly from a colloidal solution, it is 
necessary to add such an amount of electrolyte that thorough 
mixing is possible before appreciable coagulation takes place. 
From these considerations, it follows that precipitation of a 
hydrous substance as a result of double decomposition might form 
a jelly instead of a gelatinous precipitate in case the thorough 
mixing of the solutions could be effected before precipitation 
begins and in case the precipitation, once started, proceeds at a 
suitable rate. Such conditions do not obtain as a rule; but they 
are entirely possible theoretically. Thus the precipitation 


1 Weiser and Bioxsom: J. Phys. Chem., 28, 26 (1924). 


JELLIES AND GELATINOUS PRECIPITATES 29 


may be the result of a stepwise process, one step of which proceeds 
at a suitably slow rate. It is further possible to have a reaction 
that goes very slowly at low temperatures but with marked velocity 
at higher temperatures. This would not only allow of mixing 
without precipitation but would enable one to control the sub- 
sequent rate of reaction by a suitable regulation of the tempera- 
ture. Such a favorable combination of circumstances apparently 
obtains when a manganese salt of a strong acid and KH,AsO, are 
mixed. The latter salt ionizes thus: KH.AsO s K’ + H.AsOy.’; 
but on account of the solubility of Mn(H2AsO,)2, no Mn” ions 
are removed from solution by interaction with H,AsO,’. The 
latter ion, however, undergoes secondary ionization to a slight 
degree as follows: H2AsQO.’ <S H* + HAsO,’’; and _ insoluble 
MnHAsO, is formed in accord with the reaction: Mn’’ + 
HAsO,” = MnHASO,.! 

Since the precipitation of MnHAsQO, is accompanied by the 
formation of an equivalent amount of free hydrogen ion in solu- 
tion, an equilibrium is set up which prevents the complete precip- 
itation of the manganese. However, the amount of MnHAsO, 
formed and the rate of formation by the above process are influ- 
enced to a marked degree by the temperature, so that it is possible 
to obtain good jellies by mixing dilute solutions of the necessary 
salts in the cold and allowing the mixture to stand at room tem- 
perature or warming to a suitable temperature. This has been 
demonstrated with the arsenates of manganese, cobalt, iron, 
cadmium, and zinc.2, When the precipitated particles are very 
highly hydrous and when the tendency to crystallize is slight, 
very dilute jellies may be prepared by this method. Thus a 
firm jelly is formed with 0.5 per cent and soft jelly with but 0.25 
per cent MnHAsO,. A microscopic examination of this jelly 
shows that it consists of filaments or fibrils. Here again, the 
time factor is important for the formation of an enmeshing net- 
work of hydrous filaments. 

Favorable conditions for the precipitation of a jelly may be 
realized by the slow hydrolysis of a suitable salt. Thus if a 
solution of aluminum sulfate is poured on a few iron turnings, 


i Dwrez: Kolloid-Z., 14, 139 (1914). 
2 Wuiser: J. Phys. Chem., 28, 26 (1924). 


30 THE HYDROUS OXIDES 


slow hydrolysis takes place, with the ultimate formation of a 
firm hydrous aluminum oxide jelly.! 


VAPOR-PRESSURE RELATIONS 


Freshly precipitated gelatinous oxides, such as the hydrous 
oxides of iron, chromium, aluminum, tin, and silicon, have a 
vapor pressure almost the same as water and maintain it until 
the water content of the gels is lowered quite appreciably. Van 
Bemmelen? has examined a large number of such oxides and has 
found the loss of water in dry air to be continuous, the vapor- 
pressure curve showing no breaks such as would be expected if 
definite chemical compounds—hydrates—were formed. Formu- 
las for definite hydrates of precipitated oxides are frequently given 
in the literature, but in the vast majority of cases, the composition 
indicated by these formulas is purely accidental, depending as 
it does on so many factors, such as the conditions of formation, 
the method of drying, and the age.* In general, it may be said 
that the metallic oxides precipitated in a highly gelatinous form 
are never hydrates, so that they should be looked upon as 
hydrous oxides rather than hydrous hydrated oxides. This does 
not mean that there are no hydrates of the metallic oxides, for 
there are a few, among which may be mentioned Fe20;:H.O and 
Al,O3:3H20O; but, as a rule, these must be prepared in a 
special way. 

Four general types of vapor-pressure-composition curves may 
be distinguished. These are represented diagramatically in 
Fig. 1. Curve I is typical of the crystalline hydrates of which 
MoO;-2H:O is an example. Curve II likewise indicates the 
formation of a hydrate, although the steps in the curve are not 
so sharp as in curve I. The absence of sharp breaks even when 
definite hydrates are formed is accounted for by assuming that a 
part of the hydrate water is held definitely in place in the crystal 
lattice while a part may move about in the crystal lattice with 


1CuSHMANN and CoaaGEsHALL: Trans. Am. Electrochem. Soc., 39, 81 
(1921). 

2“T)ie Absorption” (1910). 

3 Weiser: J. Phys. Chem., 24, 277, 505 (1920); 26, 401, 654 (1922); 27, 
501 (1923). 

4Cf. Hitrria: Kolloid-Z., 35, 337 (1924). 


JELLIES AND GELATINOUS PRECIPITATES 31 


more or less freedom.!. Yellow tungsten trioxide WO,;-H.O 
behaves in this way. Curve III is an adsorption curve typical 
of such hydrous oxides as ferric oxide and chromic oxide in which 
the water is not held in any definite proportion. Curve IV is 
likewise an adsorption curve in which there is a break, owing 
to change in the size of the pores in the hydrous gel such as is 
observed with silica. 


rae 
aa 


Water Content 


Fig. 1.—Types of vapor-pressure curves. 


Pressure 








Vapor 


An elastic jelly, such as gelatin, loses water continuously in 
dry air, just as does a gelatinous oxide;? but, unlike the latter, the 
process is very much more nearly reversible, a dry plate taking 
up moisture and swelling again in moist air. As already pointed 
out, pores are not developed by the dehydration, as in the case 
of silica gel. A still more striking difference between the non- 
elastic gels is that the former will take up a great deal more water 
when dipped in the liquid than when suspended in the vapor at 
the same temperature. Von Schréder*® studied the behavior of 
gelatin in liquid water and in water vapor, and was led to con- 
clude that the vapor pressure of water in gelatin must be higher 
than that of pure water because water distills from the gelatin 


1Hurrie: Fortsch. Chem., Physik, physik. Chem., 18, 5 (1924). 
2 Katz: Z. Elektrochem., 17, 800 (1911). 
8 Z. physik, Chem., 45, 109 (1903). 


32 THE HYDROUS OXIDES 


to the vapor phase. Bancroft! explains von Schréder’s results 
by postulating a cellular structure for gelatin. The walls of the 
cell will adsorb a certain amount of water from the saturated 
vapor, but the microscopic cells or pockets will not be filled unless 
the gelatin is immersed in water. On lifting the swollen jelly 
into the vapor phase, water will distill from the curved micro- 
scopic droplets to the plane surface of water in the containing 
vessel, because of the higher vapor pressure of the former. As | 
Bancroft points out, the objection to this explanation is that it 
postulates a cellular structure for gelatin, which seems more and 
more improbable in the light of recent investigations. Wolff 
and Bichner? claim that water does not distill from a fully dis- 
tended gelatin jelly into the vapor phase and that von Schréder’s 
conclusions are the result of experimental error. Washburn? 
finds that moistened clays will dry in a closed vessel above water, 
a result that supports von Schréder’s observations; but he believes 
this to be due to the action of gravity. There seems no way of 
settling the question definitely except by a careful repetition of 
von Schréder’s experiments. 

Whenever a dry gel takes up moisture, heat is evolved‘ and a 
contraction in volume® takes place, particularly in the earlier 
stages. Although the volume of the system, water + dry gel, is 
greater than that of the swollen gel, the gel itself increases 
in volume and so may exert a very high pressure. In some 
experiments on dried seaweeds, Reinke® found that water was. 
taken up against a pressure or 41 atmospheres, the volume 
increase amounting to 16 per cent. Similarly, Rodewald’ found 
that starch swells against a pressure of 2500 atmospheres. Posn- 
jak? made some observations on the amount of water with which 
gelatin is in equilibrium at various pressures and on the cor- 


1“ Applied Colloid Chemistry,” 75 (1921). 

2 Koninklijke Akad. Wetenschappen Amsterdam, 17, May 30 (1914); Z. 
physik. Chem., 89, 271 (1915). 

3 J. Am. Ceram. Soc., 1, 25 (1918). | 

4 WIEDEMANN and LijpEKina: Wied. Ann., 25, 145 (1885); RopEWALD: 
Z. physik. Chem., 24, 193 (1897). 

6’ LipEKING: Wied. Ann., 35, 552 (1888). 

6 Hanstein’s botan. Abhandl., 4, 1 (1879). 

7Z. physik. Chem., 24, 193 (1897). 

8 Kolloidchem. Bethefte, 3, 417 (1912). 


JELLIES AND GELATINOUS PRECIPITATES 33 


responding behavior of raw rubber in different organic solvents. 
In all experiments the amount of liquid taken up decreases with 
increasing pressure. The data do not enable us to determine 
what pressure would be necessary to prevent any swelling or to 
remove all the adsorbed liquid from a swollen jelly; these values 
would probably be very high in every case. Some idea of the 
magnitude of the swelling pressure of gelatin may be obtained 
by coating a glass plate with gelatin which has absorbed the maxi- 
mum amount of water and observing the degree to which the 
glass plate is bent by the drying film of gelatin. The strain is 
frequently sufficient to break the plate or to pull pieces of glass 
off the surface. 

Having outlined the general properties of gels and their general 
methods of formation, we may proceed to a detailed consideration 
of the colloid chemistry of the hydrous oxides. It seems advis- 
able to start off with hydrous ferric oxide, one of the most common 
members of this class of compounds. 


1 GRAHAM: J. Chem. Soc., 17, 320 (1864). 


CHAPTER II 
THE HYDROUS OXIDES OF IRON 


Hyprovus FERRIC OXIDE 


Composition.—Hydrous ferric oxide, frequently misnamed 
ferric hydroxide, is thrown down as a highly gelatinous precipi- 
tate when an alkali is added to a solution of ferric salt that is 
not too dilute. This might be expected, since the percentage 
supersaturation preceding precipitation is relatively enormous, 
a condition favorable to the formation of extremely minute 
particles. The orientation of these particles into an enmeshing 
network that entrains water constitutes the gelatinous precipi- | 
tate. The bulky mass loses water gradually on standing and 
becomes more compact and granular. Since a small integral 
ratio between oxide and water may be realized by drying the pre- 
cipitated .oxide under suitable conditions, many hydrates have 
been described from time to time.t Tommasi? recognized two 
series of such hydrated oxides, yellow and red or brown, that 
were believed to bear an isomeric relation to each other. The 
members of the red series, obtained by precipitating a ferric salt 
with alkali, were very bulky, were soluble in dilute acids, and 
were dehydrated even by boiling water.? The members of the 
yellow series, prepared by oxidation of hydrous ferrous oxide‘ 
or ferrous carbonate, were denser than the red, were but sparingly 
soluble in concentrated acids, required a higher temperature for 


1Lerort: J. prakt. Chem., 64, 305 (1851); Pan pr St. GintuEes: Ann. 
chim. phys., (3) 46, 47 (1856); Davins: J. Chem. Soc., 19, 69 (1866); Wrrr- 
STEIN: Chem. Zentr., (1) 24, 367 (1853); ScuarFrNneR: Liebig’s Ann. Chem., 
51, 177 (1844); Brescrus: J. prakt. Chem., (2) 3, 272 (1871); Ramsay: 
J. Chem. Soc., 32, 395 (1877). 

2 Bull. soc. chim., (2) 38, 152 (1882). 

3 Davies: J. Chem. Soc., 19, 71 (1866). 

4Cf. Pumas: “Graham-Otto Lehrbuch,” 3rd ed., II, 2, 725 (1853). 

34 


THE HYDROUS OXIDES OF IRON 30 


dehydration, and retained a molecule of water even on prolonged 
boiling with water.! 

Since the brown precipitated oxide can be dehydrated by 
prolonged heating under water, it seems unlikely that it should 
form a definite hydrate. As a matter of fact van Bemmelen? 
demonstrated conclusively that the ratio of oxide to water in the 
red-brown compound depends entirely on the method of treat- 
ment; and that all the various formulas corresponding to definite 
hydrates are the accidental result of the method of drying used 
by the different investigators. Van Bemmelen attributed all 
the differences between the yellow and the brown oxide to 
differences in physical structure; but there are some grounds for 
believing that the much greater tenacity with which the yellow 
compound holds onto water between certain temperatures is 
due to the formation of a yellow hydrate. The evidence for 
this view comes chiefly from attempts to establish the 
composition of minerals. | 

In addition to the anhydrous ferric oxide, hematite, mineral- 
ogists distinguish at least five hydrates of ferric oxide: 
turgite, Fe.,03;:0.5H,O; gothite and lepidocrocite, Fe2.O;-H.O; 
limonite, Fe,03;-:1.5H.,O; xanthosiderite, Fe.20O3-2H.2O; and 
limnite, Fe,0;-3H.O. Hematite is sometimes found in nature 
as a well-crystallized substance; but the monohydrate alone 
of all the others occurs in a definite crystalline form. Ruff? 
attempted to produce hydrates synthetically by heating the red- 
brown gelatinous precipitate under pressure of 5000 atmospheres. 
Between 30 and 42.5°, he claimed to get a yellow hydrate cor- 
responding to limonite; between 52.5 and 62.5°, a yellow-red 
hydrate corresponding to gothite; and at higher temperatures, 
brick-red turgite. His synthetic preparations, however, did 
not agree with the corresponding minerals in stability; and it is 
improbable that they were identical either in physical structure 
or in the amount of water they contained. By hydrolysis of a 
solution of ferric chloride, Fischer? obtained an oxide’ whose - 


1Cf. Muck: Z. fiir Chemie, 41 (1868). 

2 Rec. trav. chim., T, 106 (1888); Z. anorg. Chem., 20, 185 (1899); ‘‘Die 
Absorption,” 70, 370 (1910). 

3 Ber., 34, 3417 (1901). 

4Z. anorg. Chem., 66, 37 (1910). 


36 THE HYDROUS OXIDES 


dehydration curve was very similar to that of a natural limonite. 
The composition of a synthetic compound, prepared in a similar 
way by Posnjak and Merwin,! approached that of a monohydrate 
at 165°; and above this critical temperature it went over rapidly 
to red anhydrous oxide, analogous to hematite. However, the 
existence of a definite temperature of inversion of limonite to 
red hematite has not been established.? 

Some years ago Posnjak and Merwin! in the Geophysical 
Laboratory made a systematic analytical, optical, crystallo- 
graphic, and thermal study of ferric oxides, which proved con- 
clusively that, even in the case of minerals, no series of hydrated 
ferric oxides exists. The only hydrate whose existence has been 
established satisfactorily is the monohydrate. This occurs in 
nature in two crystalline forms, gothite and lepidocrocite; and 
in an indefinite amorphous condition with a considerable excess 
of adsorbed water, known under the name limonite. The other 
minerals, previously considered hydrates, are either hydrous 
ferric oxide monohydrate or solid solutions of hydrous mono- 
hydrate and hydrous ferric oxide. By heating the system 
Fe.03-SO3-H2,O below 130°, a synthetic yellow crystalline mono- 
hydrate is formed which is identical microscopically with certain 
natural gothites.* This appears to be the first crystalline hydrate 
to be synthesized although, more than 30 years ago, van Bem- 
melen‘ claimed to have prepared a crystalline monohydrate by 
the action of water on sodium ferrite at 15°. Van Bemmelen’s 
so-called hydrate crystals possessed the same transparency and 
shape as the crystals from which they were derived, strongly 
suggesting that they were pseudomorphs after sodium ferrite 
rather than crystals of a definite monohydrate. Moreover, 
they decomposed and lost water below 100°, whereas the natural 
and synthetic monohydrates are prefectly stable at this 
temperature. 

Since, with but one exception, there is no definite ratio of 
ferric oxide to water, the water must be retained by the hydrous 


1Am. J. Sct., (4) 47, 311 (1919). 

* Bureau Soils, Bull. 79, 18 (1911). 

3 Posngak and Mrrwin: J. Am. Chem. Soc., 44, 1965 (1922). 

‘Van BEMMELEN and KuopsiE: J. prakt. Chem., (2) 46, 497 (1892). 


THE HYDROUS OXIDES OF IRON 37 


oxides either by adsorption, as van Bemmelen supposed, or in 
solid solution, or both. Foote and Saxton! attempted to deter- 
mine the manner in which water was held, by observing the vol- 
ume changes on freezing the precipitated oxide. They came to 
the conclusion that the water which freezes gradually with falling 
temperature is held in capillaries; while water which cannot be 
frozen is in the ‘“‘combined” state. The ‘‘combined” water is 
given up slowly on heating the precipitate with water, with no 
tendency toward simple ratios between ferric oxide and water; 
and when once dehydrated, the material does not take up water 
to form hydrates. It was assumed, therefore, that the ferric 
oxide and water ‘‘combine”’ in indefinite proportions, which is 
essentially a case of solid solution rather than of adsorption. 
The failure of gel water to freeze at a certain temperature is no 
criterion for determining whether water is dissolved or adsorbed. 
Therefore, there appears to be no justification for assuming the 
existence of a solid solution, unless it is proved that the gelati- 
nous oxide is always crystalline, and that some of the water forms 
an integral part of the crystal lattice. Some recent observations 
of Simon and Schmidt? bear on this point: 

Samples of hydrous ferric oxide of different degrees of fineness 
were prepared as follows: (1) by precipitation of a ferric nitrate 
solution with ammonia at 40°; (2) by precipitation of a cold ferric 
chloride solution with ammonia; and (3) by dialysis of a solution 
of ferric nitrate until the gel precipitated and the dialysate was 
free from nitrate. Sample 1 was relatively coarse, sample 3 
very fine, and sample 2 of intermediate fineness. The tempera- 
ture-composition curve for each preparation was obtained with a 
special tensi-eudiometer.? As shown in Fig. 2, all the curves are 
continuous and give no indication whatsoever of the formation of 
definite hydrates containing one, two, or more molecules of water. 
It was demonstrated by x-ray spectography that oxide 1 was 
completely amorphous when dried in the air, but showed amicro- 
scopic crystals of Fe20; when heated to a temperature sufficiently 
high to drive off all the water. The air-dried oxide 3, prepared 
by slow hydrolysis of a ferric salt, gave a weak interference pat- 

1J. Am. Chem. Soc., 38, 588 (1916); 39, 1103 (1917). 


2 Kolloid-Z. (Zsiqgmondy Festschrift), 36, 65 (1925). 
3 Hurria: Z. anorg. Chem., 114, 161 (1920). 


38 THE HYDROUS OXIDES 


tern! which appeared somewhat different from the x-radiogram of 
anhydrous ferric oxide. ‘This suggests that certain gelatinous 
preparations may contain some water in solid solution between 
the oriented Fe,03; molecules in the lattice. As Posnjak and 
Merwin have shown, under certain special conditions the water 





a 
Rtas 
Bac 


Mols of Water to One Mol Ferric Oxide 





0 Sl i 
Q 50 lo0 )§=6—150)=6200 36250 )= = 300 
Temperature 


550 400 


Fic. 2.—Temperature-composition curves of hydrous ferric oxides. 


molecules in the Fe2O3 lattice may assume fixed positions in the 
stoichiometrical relation represented by the definite compound, 
Fe.Q3 . H.O. 


FERRIC OXIDE SOLS 


The Péan de St. Gilles Sol.—Péan de St. Gilles? prepared 
colloidal ferric oxide by continued boiling of a solution of ferric 


1Cf., however, BOumM: Z. anorg. Chem., 149, 203 (1925). 
2Compi. rend., 40, 568, 1243 (1855). 


THE HYDROUS OXIDES OF IRON 39 


acetate. The red-brown color which is characteristic of the 
acetate becomes brick red as the boiling continues, and the 
peculiar taste of ferric salts gives place to that of acetic acid. 
The colloid is distinctly turbid in reflected light, but is perfectly 
clear in transmitted light. The red-brown deposit, formed by 
coagulation with dilute sulfuric acid, is relatively insoluble in 
even the more concentrated acids. When the colloid is poured 
into hydrochloric acid, there is formed a finely divided granular 
brick-red precipitate which shows no resemblance to ordinary 
hydrous ferric oxide. Giolitti! extended the experiments of Péan 
de St. Gilles and confirmed the important observation that the 
physical character of the precipitated colloid varies with different 
precipitating agents. The addition of a small quantity of sul- 
furous, sulfuric, selenious, iodic, periodic, boric, or phosphoric 
acid produces a gelatinous precipitate which is not repeptized 
by water, while adding a small amount of hydrochloric, hydro- 
bromic, hydriodic, nitric, perchloric, or perbromic acid causes 
slight precipitation of a finely divided brick-red powder not easily 
- removed by filtration. A larger amount of one of the latter group 
of acids precipitates completely a powder that is readily peptized 
by water, as Péan de St. Gilles observed.’ 

An investigation® of the anomalous behavior of the Péan de 
St. Gilles colloid with different electrolytes showed that the for- 
mation of a gelatinous precipitate is independent of the valence 
of the precipitating ion; and that the most voluminous precipi- 
tate is obtained when there is immediate agglomeration through- 
out the entire solution and when the precipitating agent has no 
solvent action on the particles. The conclusions accord with the 
view that a gelatinous precipitate will result whenever finely 
divided hydrous particles are thrown down under conditions 
favoring the formation of an enmeshing network enclosing liquid. 
It is well known that slow precipitation from a supersaturated 
solution is conducive to the building up of large crystals; whereas 
rapid precipitation results in very small crystals which may give 
rise to a gelatinous mass. Ina Péan de St. Gilles sol, we have the 
very finely divided particles essential to the formation of gelati- 

1 Gazz. chim. ital., 35, 11, 181 (1905). 


2Cf. Weiser: J. Phys. Chem., 24, 312 (1920). 
3 WerisER: Jbid., 24, 298 (1920). 


40 THE HYDROUS OXIDES 


nous ferric oxide; but if the sol agglomerates too slowly, the pri- 
mary particles may orient themselves into more or less granular 
masses that will entangle relatively little water. On the other 
hand, if the sol agglomerates rapidly, there is no time for the 
orientation of the primary particles, and a network may form that 
will enclose relatively large amounts of water and hence will be 
relatively voluminous. On this account, a gelatinous precipitate 
is always obtained when the precipitating anion is polyvalent, 
since low concentrations of such ions cause rapid agglomeration. 

Although substances containing anions of high precipitating 
power cause rapid agglomeration to gelatinous precipitates, this 
property is obviously not confined to electrolytes with strongly 
adsorbed anions. Thus, potassium chloride produces a very 
voluminous precipitate although chloride ion is not usually 
adsorbed strongly; whereas hydrochloric acid causes the precipi- 
tate to come down granular. With these two electrolytes the 
different character of precipitate is due to the rate of agglomera- 
tion and the solvent action of the precipitant. Potassium chlo- 
ride possesses no solvent action, and the stabilizing influence of 
potassium ion is relatively slight; hence, when the precipita- 
tion value is exceeded, rapid agglomeration takes place. On 
account of the stabilizing influence of hydrogen ion, the precipi- 
tation value of hydrochloric acid is greater than that of its 
potassium salt, and the rate of precipitation is slower. 

Contrary to the usual statement, hydrochloric acid has an 
appreciable solvent action on the Péan de St. Gilles ferric oxide, 
particularly while the oxide is in the colloidal state. Accordingly, 
the slower the sol is agglomerated by an acid, the greater will be 
the amount of oxide dissolved. Although the Péan de St. Gilles 
colloid is fairly uniform, there is unquestionably considerable 
variation in the size of the individual particles, such as Zsigmondy 
and others have observed with colloids generally; and it is the 
smallest particles that are the most readily attacked. Hence, 
the solvent action will, to a greater or lesser extent, prevent the 
formation of an enmeshing network of particles, thus cutting 
down the amount of water that can be enclosed and carried down; 
and so decreasing the volume of the precipitate. 

From what has been said, it follows that the addition of suffi- 
cient hydrochloric acid to cause rapid agglomeration, before the ~ 


THE HYDROUS OXIDES OF IRON 41 


solvent action of the acid has had time to manifest itself appreci- 
ably, should yield a more voluminous precipitate than is obtained 
in the region of the precipitation value. Furthermore, a less 
gelatinous precipitate should be obtained with a concentrated 
solution of sulfuric acid than with the very weak solution neces- 
sary to cause precipitation. Finally, if the stabilizing influence 
of the cation tends to cut down the rate of agglomeration, then 
salts with polyvalent cations and univalent anions should produce 
_a granular precipitate under certain conditions. All these con- 
clusions are readily verified experimentally. 

Scheurer-Kestner! obtained the Péan de St. Gilles colloid by 
heating a dilute solution of ferric nitrate; while Krecke? used 


TABLE J].—Hypro.ysis or Ferric ACETATE 





























Solution boiled Old solutions New solutions 
Ferric ace- Color of Color of 
tate 0.7 M eek ee Gy precipitate ene precipitate 

50.0 0.0 ||Reddish orange} Yellow Very dark red| Dark red 
25.0 25.0 |/Orange Yellow Dark red Dark red 
12.5 37.5 ||Light orange Yellow Red Dark red 
ei ar 43.7 ||Yellow Yellow Light red Dark red 
3.2 46.8 ||Canary Yellow Orange red Dark red 


ferric chloride. A pure sol may be prepared’ in a relatively 
short time by boiling a colloidal solution of ferric oxide peptized 
by the minimum amount of acetic acid. The Péan de St. Gilles 
colloid is always described as brick red, but the color varies with 
the conditions of preparation, a yellow sol being formed if the 
ferric acetate solution is allowed to stand for some time at room 
temperature before subjecting it to the boiling temperature. 
This is shown by the results recorded in Table I: A solution of 
ferric acetate approximately 3 M with respect to iron but con- 


1 Ann. chim. phys., (3) 57, 23 (1850). 

2 J. prakt. Chem., (2) 3, 286 (1871); cf. DeBray: Compt. rend., 68, 913 
(1869). 

3 WeIsER: J. Phys. Chem., 24, 299 (1920). 


42 THE HYDROUS OXIDES 


taining excess ferric oxide was allowed to stand 10 days, after 
which a 20-cubic-centimeter portion was diluted to 100 cubic 
centimeters and from it were prepared 50-cubic-centimeter por- 
tions of other solutions of various concentrations, as given in the 
table. Ina similar way, a series of solutions was made up from a 
freshly prepared solution of ferric acetate. Both sets of solutions 
were boiled vigorously on an electric hot plate for 15 hours, 
the water being replaced as it evaporated. A difference in the 
color of the two series of sols was soon noted and became quite 
pronounced as the boiling continued; the colloids from the old 
ferric acetate were yellow, and from the new red. After discon- 
tinuing the boiling, samples of each sol were precipitated with 
potassium sulfate and the color of the precipitate noted. 

Neidle! obtained a yellow colloid by dialysis of a ferric chloride 
solution in the hot; but only the sols prepared from very dilute 
solutions were stable. 

The Graham Sol. 
respects from the Péan de St. Gilles sol, by the dialysis of a ferric 
acetate solution in the cold; or more usually, by peptizing gela- 
tinous ferric oxide in ferric chloride solution and then removing 
the excess ferric chloride by dialysis. Unlike the Péan de St. 
Gilles sol, Graham’s preparation is colored deep reddish brown 
and is clear. Moreover, the coagulum obtained on adding elec- 
trolytes is highly gelatinous and is readily soluble in dilute acids. 

The Graham sol has been widely used in investigations of 

colloidal behavior and various methods have been employed in 
its preparation. Krecke* hydrolyzed ferric chloride solutions 
without dialysis. Builtz* hydrolyzed dilute solutions of ferric 
nitrate; while van Bemmelen’ used ferric chloride, and Freund- 
lich,® iron carbonyl. Grimaux’ prepared a similar product by 
pouring an alcoholic solution of ferric ethylate into water. 





1 J. Am. Chem. Soc., 39, 76 (1917). 

2 J. Chem. Soc., 15, 250 (1862). 

3 J. prakt. Chem., (2) 3, 286 (1871); cf. Wriaut: J. Chem. Soc., 43, 156 
(1883). 

4 Ber., 35, 4431 (1902). 

5 Z. anorg. Chem., 36, 380 (1903). 

6 FREUNDLICH ed Wiodwitumercee Kolloid-Z., 33, 222 Seri 

7 Compt. rend., 98, 105, 1434 (1884), 


THE HYDROUS OXIDES OF IRON 43 


Gaurilow! oxidized ferrous carbonate with hydrogen peroxide; 
and Neidle and Crombie? oxidized ferrous chloride with potassium 
permanganate in the cold and dialyzed. Neidle also oxidized 
ferrous chloride with hydrogen peroxide’ and dialyzed in the hot.‘ 
The latter method is particularly satisfactory for the rapid prepa- 
ration of colloidal ferric oxide free from metals other than iron. 
A similar sol was formed by hot dialysis of a solution of ferric 
chloride to which was added sufficient ammonium hydroxide to 
react with 60 per cent of the salt.° Tribot and Chrétien® used 
electrodialysis to obtain a pure sol fairly rapidly. The cathode 
was placed in the colloid and the anode in the water which sur- 
rounds the membrane of the dialyzer. 

Giolitti’ reports that the Graham sol approaches more nearly 
to the properties of the Péan de St. Gilles sol the longer the time 
of standing and the higher the temperature of preparation. 
Browne,® on the other hand, failed to find any appreciable differ- 
ence between old and fresh Graham sols or between cold and hot 
dialyzed sols provided the hot dialysis was not begun until the 
first large excess of electrolyte had been removed. A 4-year-old 
sol containing 40 equivalents of iron to 1 of chlorine had the same 
conductivity and stability toward electrolytes as a fresh sol of 
the same concentration and purity. 

In the preparation of the Péan de St. Gilles sol by the author’s 
method, particular care was taken to wash the hydrous oxide 
by a centrifugal process until it started to go into colloidal solu- 
tion, before peptizing with acetic acid. The process of washing 
out the precipitating agent constitutes one of the general disper- 
sion methods of making sols; but its application to the preparation 
of colloidal solutions of the hydrous oxides we owe to Brad- 
field? who worked out a centrifugal method which appears 


1 Kolloid-Z., 37, 46 (1925). 

2 J. Am. Chem. Soc., 38, 2607 (1916). 

3 J. Am. Chem, Soc., 39, 2334 (1917). 

4 J. Am. Chem. Soc., 38, 1270 (1916). 

5 NEIDLE and BaraB: J. Am. Chem. Soc., 39, 79 (1917). 

6 Compt. rend., 140, 144 (1905). 

7 GrouitTi: Gazz. chim. ital., 38, 11, 252 (1908); cf. [bid., 35, II, 181 (1905); 
36, II, 157 (1906). 

8 Private communication. 

9 J. Am. Chem. Soc., 44, 965 (1922). 


44 THE HYDROUS OXIDES 


particularly useful for preparing a pure sol in the minimum time: 
Ammonium hydroxide was added to a concentrated solution of 
ferric chloride with constant stirring until minute floccules of 
hydrous oxide were barely visible. The more granular precipi- 
tate, formed by adding an excess of ammonium hydroxide, was 
less reversible. The precipitate was allowed to settle and was 
washed by decantation until it commenced to become colloidal. 
This solution was then passed through a Sharples Laboratory 
Supercentrifuge making 32,500 revolutions per minute, at the 
rate of about 3 liters per hour. At the end of the run the sleeve 
was coated with an extremely finely divided layer of a reddish- 
brown hydrous oxide. This material was removed, mixed to 
a uniform paste with water, using a mortar and pestle; and was 
then poured into a large bulk of water for the next washing. 
After repeating the process four times, the contents of the bowl 
could be divided into two distinct parts: (1) a yellowish-brown 
fairly stiff deposit on the lowest 5 centimeters of the sleeve which 
graded slowly into (2), a dark-red highly hydrous deposit that 
was barely stiff enough to adhere to the sleeve. The two frac- 
tions were separated, shaken with a small amount of water, and 
analyzed. From these stock solutions, stable sols of any desired 
concentration could be prepared. The liquid discharged after 
the third washing was a beautiful cherry-red sol, containing but 
a minute trace of chloride. 

The Negative Sol—A century ago, Rose! observed that 
glycerin, mannite, sucrose, and glucose will prevent the pre- 
cipitation of hydrous ferric oxide on adding alkali or ammonia, to 
a solution of ferric salt. This observation was confirmed by 
Grimaux,” who attributed the solubility of the hydrous oxide to 
the formation of a negative sol stabilized by preferential adsorp- 
tion of hydroxyl ion.* Invert sugar is seven times as effective 
as cane sugar in preventing the precipitation. In this connec- 
tion, it is of interest to note that a small amount of hydrous ferric 
oxide inhibits the crystallization of cane sugar to such an extent 
that a high percentage of molasses is obtained in plant work if 


1 Ann. chim., 24, 27 (1827). 

2 GRIMAUX: Compt. rend., 98, 1485 (1884). 

3 Cf. CHATTERJI and Duar: Chem. News, 121, 253 (1921). 
4 RIFFARD: Compt. rend., TT, 1103 (1874). 


THE HYDROUS OXIDES OF IRON 45 


the raw sugar is kept in iron vessels or the clearing ‘‘chat”’ 
contains iron.! 

Robin? added ammonia to a mixture containing glycerin, 
peptone, and ferric chloride. He claimed to get a clear solution 
of ferric peptonate; but what he had was a negative ferric oxide 
sol in which both peptone and glycerin functioned as protective 
colloids. Fischer* used glycerin as a protector in preparing a 
negative sol to use for intravenous injection in arsenic poisoning. 
The ordinary positive sol cannot be employed, as it precipitates 
the negatively charged serum. A negative sol containing excess 
of both alkali and glycerin does not precipitate serum and is not 
immediately toxic to rabbits, but it met with limited success 
in intravenous injections. Dozzie> reports considerable success 
in the treatment of anemia by injection of colloidal hydrous 
ferric oxide. 

Powis® prepared a stable negative sol without a protective 
colloid by allowing 100 cubic centimeters of 0.01N ferric chloride 
to run slowly, with constant shaking, into 150 cubic centimeters 
of 0.01N sodium hydroxide. The sol was of clear brownish- 
yellow color and showed no sign of precipitation after standing 3 
weeks, although a trace of barium chloride caused immediate 
coagulation.” 

A ferric oxide sol prepared by electrical disintegration of iron 
electrodes under water® is usually positively charged; but it 
becomes less positive, neutral, and finally negative by repeated 
filtration through such substances as filter paper, glass wool, 
cotton, or sand, which are negatively charged in the presence of 
water. According to Malarski,® the reversal of charge on the 
particles is brought about by contact with the negative filtering 
media. While this explanation seems plausible, the experiments 
should be repeated to determine to what extent the properties of 


1 THorPE: “ Dictionary of Applied Chemistry,” 3, 176 (1912). 

2 Compt. rend., 101, 321 (1885). 

3 Biochem. Z., 27, 223, 238 (1910). 

4 FiscHer and Kuznitzky: Biochem. Z., 27, 311 (1910). 

5 Gazz. ospedali clin., 41, 182 (1920). 

6 J. Chem. Soc., 107, 818 (1915). 

7Cf. Keuumr: Kolloid-Z., 26, 173 (1920). 

8’ Brepia: Z. Elektrochem., 4, 514 (1898); Z. physik. Chem., 34, 258 (1899). 
® Kolloid-Z., 23, 113 (1918). 


46 THE HYDROUS OXIDES 


the sol are altered by adsorption of stabilizing ions during 
repeated filtration. 

Composition of Ferric Oxide Sol.—Since but one precipitated 
hydrate of ferric oxide exists—the yellow monohydrate—it 
seems altogether improbable that the red colloidal solutions of 
Péan de St. Gilles or Graham contain definite hydrates. The 
variation in properties between the Graham sol and the red and 
yellow Péan de St. Gilles sol is not due to chemical structure or 
the existence of hydrates, but is the result of differences in the 
size-distribution curve of the primary colloidal particles. For 
the Graham sol, the maximum is in the region of exceedingly 
small particle size, the position of the maximum shifting toward 
larger size particles as we pass to the red and the yellow Péan 
de St. Gilles sols.!. The conditions under which the sols are 
formed favor this view. Thus the percentage supersaturation 
of ferric oxide is highest for the conditions which give the Gra- 
ham sol and lowest for those which give the yellow Péan de St. 
Gilles sol. Differences in hydration of the ferric oxide sols are 
due to differences in specific surface, the Graham sols possessing 
the greatest specific surface and, therefore, the greatest amount of 
aasorbed water. If the Graham sol is impure and dilute, there 
is a gradual growth of primary particles accompanied by a decrease 
in the amount of adsorbed water. The absence of chemical 
combination between water and ferric oxide in sols prepared by 
hot dialysis was confirmed recently by means of freezing-point 
determinations carried out by Browne.? Although the colloid 
contains ferric oxide in a highly hydrous condition, the effect of 
dextrose on the freezing point of the sol showed that the water 
associated with the oxide was adsorbed, as all the water present 
in the sol acted as solvent for dextrose or for any other soluble 
substance. 

While the particles of the red sol are hydrous oxides, it is 
possible that the yellow Péan de St. Gilles sol may contain par- 
ticles of monohydrate or of hydrous monohydrate. . 

Colloidal ferric oxides can be prepared fairly free from electro- 
lytes but it has been demonstrated repeatedly that at least some 


1 ZsIGMONDY: ‘‘Chemistry of Colloids,”’ translated by Spear, 163 (1917). 
2 J. Am. Chem. Soc., 45, 297 (1923). 


THE HYDROUS OXIDES OF IRON 47 


electrolyte must be present in such sols to ensure their stability.! 
Thus the Graham sol, peptized by ferric chloride or hydrochloric 
acid, always contains traces of chlorides, however long the dialysis 
may be continued.* On this account, a number of investigators 
consider the various dialyzed colloids to be chlorides of condensed 
ferric hydroxides like Feo(OH).14 9Fe:Cle or as oxychlorides of 
variable composition.*? This conception of the nature of col- 
loidal solutions meets with serious objection at the outset, since 
Fischer* and others’ have shown that definite chemical oxychlor- 
ides of iron do not exist at ordinary temperatures. Naturally, 
investigators who assume the existence of such definite compounds 
in ferric oxide sols are unable to agree on their composition. 
Thus, Nicolardot claims that the sols are made up of mixtures of 
two compounds in which the ratios of iron to chlorine in equiv- 
alents are 6 and 125, respectively. Neidle showed these ratios 
to be purely accidental; but believes there is a compound in 
which the ratio is 21. Recently Thomas and Frieden have 
arrived at the conclusion that 1 mol of ferric chloride is necessary 
to keep 21 mols of ferric oxide (ratio of iron to chlorine in equiv- 


1 KastTNER: Ann. chim. phys., (3) 57, 231 (1859); DeBray: Compt. rend., 
59, 174 (1864); MaGnier DE LASouRCE: [bid., 90, 13852 (1880); Hanrzscu and 
Descu: Liebig’s Ann. Chem., 323, 28 (1903); LinpER and Picton: J. Chem. 
Soc., 87, 1920 (1905); Wyrousorr: Ann. chim. phys., 7, 449 (1905); Rumr: 
Z. anorg. Chem., 48, 85 (1905); Nuiwue: J. Am. Chem. Soc., 39, 2334 (1917). 

2 Cf. Urrr: ‘Uber kolloides Eisenoxyd.,’’ Dissertation, Dresden (1915). 

3 WyRoOuUBOFF and VERNEUIL: Bull. soc. chim., (8) 21, 187 (1899); Jorpis: 
Z. anorg. Chem., 35, 16 (1903); Z. Hlektrochem., 10, 509 (1904); Ductaux: 
Compt. rend., 138, 144, 809 (1904); 140, 1468, 1544 (1905); 143, 296, 344 
(1906); J. chim. phys., 5, 29 (1907); LinpER and Picron: J. Chem. Soc., 
87, 1919 (1905); Nrcouarpot: Ann. chim. phys., 6, 334 (1905); Maurirano: 
Compt. rend., 139, 1221 (1904); 140, 1245 (1905); 141, 660, 680 (1905); 143, 
172, 1141 (1906); Z. physik. Chem., 68, 232 (1910); Maurirano and MIcHEL: 
Compt. rend., 145, 185, 1275 (1907); MicuEu: Compt. rend., 147, 1052, 1288 
(1908); Dumanskt: Kolloid-Z., 8, 232 (1911); Neipuu: J. Am. Chem. Soc., 
39, 2334 (1917); Maruua: Kolloid-Z., 21, 49 (1917); Tuomas and FRIEDEN: 
J. Am. Chem. Soc., 45, 2522 (1923); Pautrand Roaan: Kolloid-Z., 35, 131 
(1924); Pautiand WautsErR: Kolloidchem. Bethefte, 17, 256 (1923); Ktunu 
and Pauuti: Jbid., 20, 319 (1925). 

4Z. anorg. Chem., 66, 38 (1910). 

5 CAMERON and Rosinson: J. Phys. Chem., 11, 690 (1907); Groxirtt: 
Gazz. chim. ital., 36, 1157 (1906); Smrru and Griesy: J. Am. Pharm. Assoc., 
12, 855 (1923). 


48 THE HYDROUS OXIDES 


alents = 42) dispersed in the colloidal condition, irrespective 
of the concentration of the sol. These observations and conclu- 
sions are not in accord with Neidle, who showed that the maxi- 
mum purity obtainable before precipitation sets in increases 
appreciably with decreasing iron content. Neidle prepared a 
sol, approximately 0.05 N with respect to iron, in which the ratio 
equivalents Fe:--: equivalents Cl’ was 84; while the maximum 
purity obtained by Thomas and Frieden at this concentration 
was only about half as great. Bradfield! prepared a ferric oxide 
sol by washing by the use of the centrifuge, in which the ratio 
was 396. In the purest sol Ufer? was able to prepare by dialysis, 
the ratio was approximately 2700. 

In the light of the wide variation in the iron: chlorine ratio 
obtained by different investigators, there seems no room to doubt 
but that, within reasonable limits, the composition of a dialyzed 
colloid depends upon the condition of formation. Everybody 
knows that precipitation of a colloid will take place if the dialysis 
is carried too far; and that consistent results can be obtained only 
by a very careful control of the experimental conditions. Since 
the method of procedure followed by different investigators is 
likely to vary widely, we might expect the wide variation in the 
results which the records show. Some people require very little 
evidence to convince them of the existence of chemical compounds 
and these assign definite formulas to the dialyzed sol. Others 
who have observed the passing of many cherished and time- 
honored ‘‘compounds” content themselves with postulating the 
formation of a series of “‘indefinite’”? compounds in order to explain 
their observations. This course is of questionable value because 
of the complexity and variability of the systems that are encoun- 
tered and the consequent complexity of the hypothetical com- 
pounds that must be assumed to exist. 

Recently, attempts have been made to determine the number 
of molecules in a single collodial particle per unit of electrical 
charge, from electrical conductivity and transport measurements 
on the sols themselves and on the ultrafiltrates from the 


1J. Am. Chem. Soc., 44, 965 (1920). . 
2 “Uber kolloides Eisenoxyd.,’’ Dissertation, Dresden (1915); cf. FrmuNp- 
LicH; ‘‘Kapillarchemie,” 511 (1922). 


THE HYDROUS OXIDES OF IRON 49 


sols.1. According to Duclaux? the specific conductance K,, of the 
colloidal particles or micelles of a sol is given by the expression 


ee eee 


where K, and K; are the specific conductances of the sol and 
the ultrafiltrate from the sol, respectively. If a quantity of 
electricity H, is passed through the sol, the part carried by the 
micelles E,, will be 


nie als (1) 


If one knows the mass of the micelle ions m transferred by a 
quantity of electricity H,, together with the mobility of the 
micelle ion U,. and of accompanying anion U,, one can calculate 
the charge on the mass Ae corresponding to the electrochemi- 
eal equivalent, from the equation, 


Ae = Gems (2) 








where F is 1 faraday. Substituting for H,, its value from Eq. (1), 
Kq. (2) becomes 
FK, 4, UetU; 


Ae = RK mM 9 (3) 








If EH, is made identical with F, then m becomes S, the mass of 
the micelle ions in mols of dispersed substance, Fe2.O3. Ae thus 
becomes A, and since 











Ut Us _utv_A 
Ol. Bie ip U 
Eq. (8) may be written 
SK,.A 
Ap. = eer (4) 


Representing the sum of the mobilities of the cation and anion 
from conductivity measurements in the usual way by wu + 0; 


1 WiInTGEN: Z. physik. Chem., 103, 250 (1922); Wintcamn and BI.rTz: 
Ibid., 107, 403 (1923); WintGEN and Léwentuat: I[bid., 109, 378 (1924); 
cf. Putacart: ‘First American Congress of Chemistry” (1924); Chem. 
Abstracts, 19, 1518 (1925). | 

2 Compt. rend., 140, 1468, 1544 (1905). 


50 THE HYDROUS OXIDES 


the molecular weight of the colloidal component, Fe2O;, by M; 
and the weight in grams of Fe2QOs in a liter of sol by g, then Ae, 
which becomes Az, is 


Mieoee Thee lt) 


% i 1000. (5) 





Wintgen has developed this formula from Kohlrausch’s law of 
the independent migration of the ions, in the following way: For 
a ferric oxide sol prepared by dialysis of ferric chloride, let M = 
molecular weight of Fe.203; g = concentration in grams of Fes,Os; 
per liter; Z = number of elementary charges carried by 1 micelle; 
and n = number of FeO; molecules in 1 micelle. The electro- 
chemical equivalent weight W of the micelle is given by 


nM 


wae 6) 





which corresponds to the electrochemical equivalent weight of 
an ordinary ion; and so gives the weight in grams of Fe.0; 
which carries 1 faraday. The value 


Ae=—=5 : (7) 


gives the number of mols of Fe,O3 in a micelle ion carrying 1 
faraday. The equivalent concentration C,. of the micelles is 
then 


Cee — es (8) 


If uw and v are the mobilities of the micelle ion and chloride ion, 
respectively, from conductivity measurements, then 


1000Km = (u+0)Cae = (u +0) Ee (9) 
from which 
_ gu +2) 
Wi aan (10) 


Substituting for W its value MAe from Kq. (7), Eq. (10) becomes 


ono 
A, = Ae = 77 Top0K, - 


THE HYDROUS OXIDES OF IRON ol 


The value Ae is spoken of as the ‘‘equivalent aggregate” or as 
the electrochemical equivalent of the micelle. If, in a liter 
of sol, there are m; mols of Fe.O3 and mz gram atoms of chlorine 
(ionized and combined); and if the concentration of chlorine in 
the intermicellar liquid (that is, in the ultrafiltrate) is [Cl,], then 
the gram atoms of chlorine corresponding to Ae mols of FeO; are 


Te ne [Cli] 


my 


-Ae 





Of this, unit amount is split off from the micelle and the remainder 
E is a part of the micelle; therefore 
ma —(Ch] | 


mi 


ase Ae = 1 

Using these formulas, Wintgen! calculated the composition of 
the micelles of a number of sols. For example, assuming the 
micelles of an aged iron oxide sol (containing 1.601 grams 
Fe.O3; and 0.06014 gram Cl in 100 grams of sol) to be all the same 
size, the average composition of the micelles is represented by 
the formula 


(175.35F e203; 7.86HCI1; zH20 FeO ) 10,230 a 10,2380Cl’ 


Lottermoser? found the hydrogen ion concentration of the ultra- 
filtrates from an aged sol to be the same as that of the sol. The 
micelles are believed to contain neutral chloride as well as chloride 
ion. Since the positive charge on the particles is probably due to 
stronger adsorption of hydrogen ion than of chloride ion, their 
composition may be represented by the general formula 


(| #F e203: yHCl-2H.0 |H’)a:(n — g)Cl’ 


where gCl’ represents the chloride ion corresponding to the 
excess of adsorbed hydrogen ion to which the particle owes its 
free charge. Lottermoser* found the specific conductance of 
the sol to be higher than that of the ultrafiltrates, the difference 
being regarded as the true conductivity of the micelles. If 


1 WINTGEN and LOWENTHAL: Z. phystk. Chem., 109, 378 (1924). 
2 Z. Elektrochem., 30, 391 (1924). 
3 Cf. KopaczEwskI: Compt. rend., 179, 628 (1924). 


oO2 THE HYDROUS OXIDES 


the micelles P are considered to be complex electrolytes, the 
equivalent conductivity at infinite dilution may be calculated 
from the equation 


_ 1000K> 
Pico a Ka 





The mobility of the micelle was found to rise abnormally with 
increasing dilution in purified sols containing but small amounts 
of chlorine. This fact necessitates the assumption that the 
micelles are adsorption complexes, the abnormality being due 
to the displacing of the adsorption and hydrolysis equilibria by 
dilution. The value Ap., approaches a constant value only in 
sols rich in chlorine. From this, it would appear that nothing is 
gained by looking upon ferric oxide sols as electrolytes with 
complex cations. But if one insists on regarding them in this 
light, there is no particular objection provided one recognizes 
clearly that there is a fundamental difference between sols and 
non-colloidal, complex electrolytes. In the latter, there exists 
a simple, stoichiometric ratio between the neutral component 
and the complex-forming ion; while in sols, the ratio of neutral 
constitutent to what Lottermoser calls sol-forming ions is indefi- 
nite and changes continuously. 

A simpler and probably quite as exact an interpretation of the 
variable properties and composition of the sols may be given from 
the point of view of specific adsorption: Any number of hydrous 
ferric oxides are possible, differing among themselves in the size 
of the particles, and hence in the amount of salt or ion adsorption. 
The colloid prepared by the Graham method is formed in the 
presence of ferric chloride, hydrochloric acid, ferric ions, hydro- 
gen ions, and chloride ions.! Accordingly, we might expect the 
colloidal particles to adsorb some ferric chloride and hydrochloric 
acid, and they will always adsorb ferric, hydrogen, and chloride © 
ions in amounts depending on the nature of the colloid, the specific 
adsorbability, and the concentration.2 Now it is well known 
that a substance always shows a strong tendency to adsorb its 
own ions, and hydrogen ion is usually very strongly adsorbed; 
on the other hand, chloride ion is not usually adsorbed so 


1 BROWNE: J. Am. Chem. Soc., 45, 297 (1928). 
2 Cf. Marria: Kolloidchem. Bethefte, 3, 85 (1911). 


THE HYDROUS OXIDES OF IRON 53 


strongly, and this preferential adsorption results in a stable 
positive colloid. Since there is an equilibrium between the 
amount of a substance adsorbed and the amount in solution, 
prolonged dialysis will result in the loss of part of the adsorbed 
cations (together with an equivalent amount of anions) and this 
will decrease the stability of the sol. Adsorbed chloride, either 
as salt or as ion, will not give a test with silver nitrate; and small 
amounts of unadsorbed chloride in the presence of colloidal iron 
oxide cannot be detected by precipitation with silver nitrate, 
since the protecting action of the hydrous oxide does not allow the 
particles of silver chloride to become large enough to cause tur- 
bidity.!. Moreover, adsorbed chloride will have a negligible 
effect on a chlorine electrode and will not be detected potentio- 
metrically; hence it is not surprising to learn that the amount of 
chlorine as ion is less than the total chlorine content of the sol.” 
From this point of view, it is obviously unnecessary to postulate 
the existence of oxychlorides of varying composition to account 
for the observation that only a part of the chlorine present 
appears to exist as ion.? 

Since colloidal solutions, in general, are instable in the absence 
of some soluble substance that is strongly adsorbed by the col- 
loidal particles, it follows that a colloidal solution will show a 
slight osmotic pressure and freezing-point lowering. This has 
been observed by a number of investigators with Graham’s 
colloidal ferric oxide. Duclaux‘* found that the osmotic pressure 
increases with the concentration of sol but is not proportional 
to it. He demonstrated also that the osmotic pressure falls 
off slightly with rise in temperature, a result that was confirmed 
by Zsigmondy.®> Both Duclaux and Malfitano® observed that 
the osmotic pressure of ferric oxide sols does not vary directly 
with their conductivity, the latter decreasing more rapidly than 
the former with dilution of the sol. It has been customary to 
interpret these results qualitatively by postulating the presence 


1RuER: Z. anorg. Chem., 48, 85 (1905). 

2Pauti and Matuta: Kolloid-Z., 21, 49 (1917) 

3 See DumanskI: Kolloid-Z., 8, 232 (1911). 

4 Compt. rend., 140, 1544 (1905). 

5 ““Chemistry of Colloids,’ translated by Spear, 167 (1917). 
6 Compt. rend., 189, 1221 (1904). 


54 THE HYDROUS OXIDES 


in the sol of complex oxy-salts having all the necessary properties;! 
but such an explanation is not particularly helpful. 

If we have a suspension that is altogether insoluble and con- 
tains no impurities, it will give rise to no osmotic pressure. The 
osmotic pressure of a well-dialyzed Graham sol is due partly to 
the colloidal particles which have adsorbed ions; but chiefly to 
the ions of ferric chloride and hydrochloric acid. Since the 
behavior of an adsorbed ion will depend on the size and nature 
of the adsorbing particle, it follows that any factor affecting the 
physical character of the particles or the adsorption of ions by 
them will influence the osmotic pressure of the sol. Moreover, 
since the osmotic pressure and freezing-point lowering in a well- 
purified sol are necessarily small, molecular weights deduced there- 
from may be absurdly large.” In the nature of things, it is wrong 
to attribute the observed osmotic pressure and freezing-point 
lowering of any sol to the insoluble suspended material, and 
molecular weights deduced from such data are meaningless. 
The experiments of Duclaux and Malfitano should be repeated, 
and observations made of the effect of dilution and temperature 
on the number, physical character, adsorbability, and mobility 
of the colloidal particles of hydrous oxide. 

Optical Properties—Majorana* made the interesting observa- 
tion that a sol exhibits pronounced double refraction when 
placed in the field of a powerful electromagnet and traversed by 
a light ray at right angles to the lines of force. This property 
is undoubtedly due to orientation of the particles of sol by the 
electric field;+ for this orientation and the concomitant double 
refraction can be observed directly by working with a sol con- 
taining particles large enough to see with an ordinary microscope. 
Moreover, a gel formed by coagulation of a sol in an electric 
field exhibits permanent double refraction, whereas coagulation 
under ordinary conditions gives an optically inactive gel. Large 

1 MauriTano: Compt. rend., 139, 1221 (1904); Ducuaux: J. chim. phys., 
7, 405 (1919). . 

2TLINDER and Picton: J. Chem. Soc., 87, 1920 (1905); Dumanskr1: 
Kolloid-Z., 8, 232 (1911). | 

3 Atti accad. Lincet, 11, (1) 374, 463, 531; 12, (1) 90, 139 (1902). 

4Scumauss: Drude’s Ann., 12, 186 (1903); CoTtron and Movuron: 


Compt. rend., 141, 317, 349 (1905); ‘“‘Les Ultramicroscopes,’” Paris, Chap. 
VIII (1906). 


THE HYDROUS OXIDES OF IRON 55 


particles cause a greater effect than small ultramicrons since the 
Brownian movement of the latter prevents sufficient orientation 
to cause pronounced double refraction. Cotton and Mouton 
attribute the optical phenomenon to the particles themselves 
and not to their position alone. This is in accord with Freund- 
lich’s! observation that colloidal solutions of ferric oxide and 
vanadium pentoxide” showing the Majorana phenomenon exhibit 
double refraction when stirred mechanically* or when a current 
of electricity is passed through the sols. These observations 
lend support to Nageli’s* view that the particles of certain sols 
consist of anisotropic ultramicrons having a resemblance to 
tiny crystals. 


THE PRECIPITATION OF SOLS BY ELECTROLYTES 


Investigations on the precipitation of ferric oxide sols by elec- 
trolytes have been confined pretty largely to Graham’s sol. 
Duclaux, working with a colloid of this type containing 203 x 
10-® equivalents of iron and 16.6 < 10~° equivalents of chlorine 
per 10 cubic centimeters, found the critical coagulation concen- 
tration of sodium sulfate, citrate, chromate, carbonate, phosphate, 
hydroxide, and ferrocyanide to vary from 13 X10~° in the case 
of ferrocyanide to 19 X10~°® equivalents in the case of phosphate. 
These observations were believed to show that equivalent 
amounts of the various ions cause the same effect, and further- 
more, that the amount necessary for precipitation is the same 
as the chloride content of the colloid, within the limits of the 
experimental errors. He thus came to regard the precipitation 
process as a definite stoichiometric chemical action, a double 
decomposition of the ordinary type. A marked variation from 
the equivalence rule was observed with sodium chloride and sodium 
nitrate which required 2000 < 10-* and 1880 * 10~° gram equiv- 
alents, respectively, to precipitate the same amount of colloid 
as the seven salts above referred to. Freundlich® found a wide 


1Z. Elektrochem., 22, 27 (1916). 

2 See p. 266. 

3 Cf. QuINcKE: Drude’s Ann., (4) 15, 28 (1904); Treri: Atte accad. 
Lincet, (5) 19, 470 (1910). 

4 “Theorie der Garung,’”’ Miinchen, 121 (1879). 

® “ Kapillarchemie,”’ 352, 358 (1909). 


56 THE HYDROUS OXIDES 


variation from equivalence in the precipitation concentration of 
various salts, which he attributed to a difference in the adsorb- 
ability of the precipitating anions. From Freundlich’s observa- 
tions the order of precipitating power of the anions is: dichro- 
mate > sulfate > hydroxide > salicylate > benzoate > chloride > 
nitrate > bromide > iodide. 

The results of some experiments! on the precipitation of a 
Péan de St. Gilles sol (1.54 grams Fe,O; per liter) with various 
potassium salts give the following series, beginning with ferro- 
cyanide which has the greatest precipitating power: ferrocyanide 
> ferricyanide > dichromate >tartrate>sulfate>oxalate> 
chromate > iodate > bromate > thiocyanate > chloride > chlorate > 
nitrate > bromide>iodide. As one should expect, the order of 
ions is identical with that deduced from Freundlich’s data for 
the ions common to both series. Owing to differences in nature 
and purity? of the Péan de St. Gilles sol and Duclaux’s sol, the 
precipitation concentrations of electrolytes were higher for the 
former than for the latter; but the magnitude of the variation is 
relatively unimportant compared with the fact that the precipita- 
tion values are not the same, as Duclaux believed. The Graham 
sol is stabilized by preferential adsorption of hydrogen ion and 
probably some ferric ion. The unadsorbed chloride ion present 
in an extremely pure sol is a measure of the excess cationic 
adsorption which gives the colloid particles their charge and the 
colloid its stability. Different concentrations of electrolytes are 
necessary to neutralize the adsorbed ions and precipitate the sol. 
The concentration of anion necessary to effect neutralization will 
approximate the chloride ion concentration only in so far as its 
adsorption tendency approaches that of the adsorbed cations. 
The precipitation concentrations of acids are uniformly higher 
than those of potassium salts since the stabilizing hydrogen ion 
is more strongly adsorbed than potassium ion. The order of 
precipitating power of electrolytes changes but little with varia- 
. tion in the hydrogen ion concentration of the sol.* 


1 WEISER and MippietTon: J. Phys. Chem., 24, 641 (1920). 

2See p. 91 for a discussion of the influence of purity of sols on the pre- 
cipitation values. 

3 Rona and Lipmann: Biochem. Z., 147, 163 Wb 


To 


€ 
. 
| 





THK HYDROUS OXIDES OF IRON o7 


The change in dispersity of ferric oxide during coagulation 
does not involve a measurable heat effect. The precise investiga- 
tions of Mathews and Browne! show that the heat effects during 
precipitations of sols of low purity, are due to dilution of the 
ferric chloride and hydrochloric acid in the sols; to mixing of 
these electrolytes with the coagulating electrolyte; and to changes 
in the adsorption equilibria. The absence of heat effect on 
coagulation indicates either (1) a very low interfacial tension 
between hydrous ferric oxide and water, or (2) no appreciable 
change in specific surface during coagulation. In support of 
the latter viewpoint Bradfield? showed that so-called “irreversi- 
ble” coagula could be repeptized by thorough washing in the 
centrifuge. Apparently, coagulation by electrolytes is not 
accompanied by a growth of the primary colloidal particles, but 
the latter merely agglomerate into loose clumps without occa- 
sioning any marked decrease in the specific surface. 

Effect of Concentration of Sol.—The early observations of 
Freundlich? on the precipitation of colloidal arsenious sulfide 
led him to the erroneous conclusion that the precipitation values 
of electrolytes for colloids of different concentrations bear a 
constant ratio to each other. Thus Kruyt and van der Spek? 
found that the precipitation value of potassium chloride for 
colloidal arsenious sulfide increases, and of aluminum chloride 
falls off, with decreasing concentration of colloid; while the 
precipitation value of barium chloride does not change appreci- 
ably with the dilution. Similar results obtained by Burton and 
Bishop® with colloidal arsenious sulfide and mastic® led to the 
formulation of the following rule: The precipitating action of 
univalent ions increases, that of divalent ions remains unchanged, 
and that of trivalent ions decreases with diminishing concentra- 


1J. Am. Chem. Soc., 43, 2336 (1921); Browne: Ibid., 45, 297 (1923). 

2 J. Am. Chem. Soc., 44, 965 (1922). | 

3 Z. physik. Chem., 44, 129 (1903). 

4 Kolloid-Z., 25, 3 (1919); cf. MukHopapHyaya: J. Am. Chem. Soc., 37, 
2024 (1915). 
5 J. Phys. Chem., 24, 701 (1920); Burton and MaclInnus: [bid., 25, 517 
(1921). ’ 

6 Cf. Neisser and FrrepEMANN: Mtinch. med. Wochenschr., 51, 827 (1904); 
cf. Becuuo.p: Z. physik. Chem., 48, 385 (1904); Bacu: J. chim. phys., 18, 52 
(1920). 


08 THE HYDROUS OXIDES 


tion of sol. Some investigations! were carried out in the author’s 
laboratory using colloidal chromic oxide, Prussian blue, Péan de 
St. Gilles ferric oxide, and arsenious sulfide. The results of a 


a 


ZB 
e 
3 
EF 


evel 


Ratio of Precipitation Values 


0 
_ Concentration of Colloid, per cent 
Fig. 3.—Precipitation of colloidal hydrous ferric oxides. 









er a 
Batt aa 
[ a 






Eee 
E eke eee 
rc aa Yaa et 
- 0.9 SSS Seal | eee 
s | [| |e) 
2 
£07 ae 
:| | | aa 
oO 
ers ee) 
03 
0 35 50) te 100 


Concentration of Colloid, per cent 


Fic. 4.—Precipitation of colloidal arsenious sulfide sols. 


series of experiements on colloidal ferric oxide and arsenious 
sulfide are given in Table II and shown graphically in Figs. 3 
and 4. The concentrations of the sols are expressed in per 


1 WEISER and Nicuouas: J. Phys. Chem., 25, 742 (1921). 


— 


a Gt PORT. CE ip bere hts a Saag ot gg rh ONT ee Oe AS 


é 





THE HYDROUS OXIDES OF IRON 59 


cent, taking the most concentrated as 100 per cent. The curves 
in the figures were obtained by plotting concentration against 
ratio of each precipitation value for a given electrolyte to that 
of the strongest sol. In the case of ferric oxide sol, the effect of 
dilution on the precipitation value of electrolytes is clearly not 
in accord with Burton and Bishop’s rule. ! 


TaBLeE II 
Precipitation of Ferric Oxide Sols 


Concentration of colloid, | Precipitation values of 


econ’ | KBrO; K,80. K.Fe(CN), 
100 40.1 0.68 0.57 
(1.7 grams per liter) 
50 34.4 0.41 0.30 
Zo 28 .0 0.25 0.16 
1255 25.0 0.16 0.08 


Precipitation of Arsenious Sulfide Sols 





; . Precipitati ] t 
Concentration of colloid, recipitation values 0 





Noa | KCl BaCl: AICI; 
100 68.3 1.940 0.513 
(6.24 grams per liter) a 
75 5; 68.3 1.877 0.473 
50 70.0 1.800 0.380 
25 76.7 1.733 0.333 
10 80.0 1.683 0.260 





Similarly with colloidal Prussian blue? and chromic oxide, the 
precipitation value of all electrolytes diminishes as the concen- 
tration of the colloid falls off irrespective of the valence of the 
precipitating ion. In the case of colloidal arsenious sulfide, the 
precipitation value of potassium ion increases and of aluminum 
ion decreases with dilution of the sol, in accord with Burton and ° 


1 Burton and Bisnop: J. Phys. Chem., 24, 701 (1920). 
2 Of., however, Guosu and Duar: J. Phys. Chem., 29, 663 (1925). 


60 THE HYDROUS OXIDES 


Bishop’s rule. However, from the slope of the curve in Fig. 
2, it is obviously incorrect to say that the precipitating action 
of divalent barium ion is independent of the concentration of 
the colloid; this is no more true of barium ion than of potassium 
ion. 

According to Kruyt and van der Spek, two factors determine 
the effect of dilution of a colloid on the precipitation value of 
electrolytes: First, the smaller number of particles requires less 
electrolyte to lower the charge on the particles to the point of 
agelomeration; and, second, the greater distance between par- 
ticles making collision less probable, a further reduction in 
particle charge must be effected through the addition of more 
electrolyte. Since these two factors have opposite effects on 
the precipitation value, it is only necessary to assume the pre- 
dominating influence of one or the other in order to account for 
the results in a given case. Thus, Kruyt and van der Spek 
assume that the predominating influence in the precipitation 
of arsenious sulfide with potassium ion is the changing chance of 
collision, while the more important factor in the precipitation of 
ferric oxide with chloride ion is the alteration in the required 
amount to be adsorbed. The difference in behavior with pre- 
cipitating ions of the same valence is attributed to the lyophile 
properties of hydrous ferric oxide. 

Although both of the factors recognized by Kruyt and van 
der Spek unquestionably have an influence in determining the 
effect on the precipitation value of changing the concentration 
of sol, it would seem that these factors alone are inadequate to 
account for all the experimental results. The explanation sug- 
gested for the difference in behavior of colloidal arsenious sul- 
fide and hydrous ferric oxide with univalent precipitating ions 
is of doubtful value, particularly since mastic emulsion! behaves 
much like colloidal arsenious sulfide although certainly possessing 
more lyophile properties than Péan de St. Gilles’ ferric oxide. 
Furthermore, if the decreased chance of collision is the pre- 
dominating factor in preventing a weaker arsenious sulfide sol 
from coagulating in a given time in the presence of enough potas- 
- sium chloride to coagulate a stronger sol, it would seem that 


1 NEISSER and FRIEDEMANN: Mtinch. med: Wochenschr. 61, 827 (1904); 
Burton and Bisuop: J. Phys. Chem., 24, 701 (1920). 


+ i _——le 


THE HYDROUS OXIDES OF IRON 61 


complete coagulation of the weaker sol should result if sufficient 
time were allowed. As a matter of fact, however, enough potas- 
sium chloride to precipitate in 2 hours a colloid containing 5 
grams per liter will not precipitate a colloid one-fourth as strong 
in several weeks. Other observations indicate that Kruyt and 
van der Spek attach too much importance to the decreased chance 
of collision of the particles resulting from dilution of sols. Thus, 
everyone finds that the precipitation concentration varies almost 
directly with the concentration of sol in case the precipitating 
ion is of high valence. 

That the theory of Kruyt and van der Spek should be inade- 
quate in certain respects might be expected, since these investi- 
gators concerned themselves only with the precipitating ions of 
electrolytes, disregarding entirely the effect of adsorption of the 
stabilizing ions having the same charge as the colloid. If there 
is no adsorption of the stabilizing ion and if the adsorption of the 
precipitating ion is very great, there will be a tendency for the 
precipitation concentration to vary directly with the concentra- 
tion of the sol. On the other hand, if the stabilizing ion is 
adsorbed, a greater concentration of precipitating ion will be 
required to produce coagulation. This effect will be more pro- 
nounced the greater the dilution of the sol since the decreased 
chance both of collision and of coalescence will combine to render 
the sol proportionately more stable, so that correspondingly 
more of the precipitating ion must be added for complete precipi- 
tation. These conclusions are in accord with experimental 
results. 

With electrolytes sae multivalent precipitating ions, the 
influence of the stabilizing ion is frequently very small, since the 
adsorption is so slight at the very low precipitation coreenteenienn 
Under these conditions, the precipitation value diminishes to a 
greater or lesser extent as the concentration of the colloid 
decreases. As might be expected, the greater the valence of the 
precipitating ion and hence the lower the precipitation value, 
the more nearly we find the latter varying directly with the 
concentration of the sol. 

With electrolytes having univalent precipitating ions, the 
precipitation value is usually quite large. Although this is 
generally attributed to weak adsorption of the precipitating ion, 


62 THE HYDROUS OXIDES 


at the high concentration necessary for coagulation, the adsorp- 
tion of the stabilizing ion cannot be disregarded. In fact, if 
the adsorption of the two ions is of the same order of magnitude, 
both may be taken up fairly strongly and a high precipitation 
value will result. Considerable experimental evidence! indicates 
that potassium ion and lithium ion are fairly strongly adsorbed 
by arsenious sulfide, the high percipitation value of potassium 
chloride or lithium chloride for this colloid arising from appreci- 
able adsorption of chloride ion. On the other hand, the high 
precipitation value of potassium chloride for colloidal hydrous 
ferric oxide is due to relatively weak adsorption of the precipi- 
tating ion, the stabilizing ion having much less effect than with 
colloidal arsenious sulfide.? In general, it may be said that the 
adsorption of the stabilizing ion varies widely but is never 
negligible for electrolytes which precipitate only in the high 
concentration characteristic of uni-univalent electrolytes. This 
adsorption of the ion having the same charge as the sol renders 
the latter more stable, and proportionately more of the precipi- 
tating ion is required for coagulation than in those cases where 
the influence of the stabilizing ion is negligible. Under these 
- conditions we may expect the precipitation value to fall off much 
less sharply or even to increase as the colloid concentration is 
reduced, the increase being greater the higher the valence of 
the stabilizing ion. 

Mutual Precipitation of Sols.—Biltz’? investigated the pre- 
cipitation’ of positive sols, including hydrous ferric oxide by 
negative colloids, such as platinum, selenium, silica, stannic 
oxide, molybdenum blue, Mo;Os, tungsten blue, W203, and the 
sulfides of arsenic, antimony, and cadmium. Complete precip- 
itation occurs when a sol of one sign is neutralized by adsorption 
of an amount of colloid carrying an equivalent quantity of ion 
of opposite sign. The amount of various colloids necessary to 
effect mutual precipitation will depend on their nature. Thus a 
certain colloidal ferric oxide is more effective than cerium oxide 
and less effective than thorium oxide in precipitating colloidal 
gold; while both thorium oxide and cerium oxide are more effec- 

1 WEISER: J. Phys. Chem., 25, 665 (1921); Ibid., 28, 232 (1924). 


> WeIsER: Loc. cit.; cf. FREUNDLICH: Z. physik. Chem., 44, 157 (1903). 
’ Ber., 37, 1095 (1904). | 


te Oe 


ote 








THE HYDROUS OXIDES OF IRON 63 


tive than ferric oxide in precipitating colloidal antimony sulfide 
and arsenious sulfide. Similarly, a red colloidal gold, prepared 
by reduction of gold chloride with formaldehyde, requires for 
complete precipitation considerably less of a given ferric oxide 
sol than a blue-gold sol prepared with phosphorus as reducing 
agent.' Recently, Freundlich and Nathanson? found colloidal 
arsenious sulfide sol and Oden’s sulfur sol to be instable in the 
presence of each other. Since both sols are negatively charged, 
this instability cannot be due to neutralization by adsorption, 
but was found to result from interaction between the stabilizing 
agents of the two sols, hydrogen sulfide and pentathionic acid. 
This observation led Thomas and Johnson* to attribute the 
mutual precipitation of sols of opposite sign to chemical inter- 
action of the stabilizing electrolytes in the sols. Thus, the 
precipitation of Graham’s colloidal ferric oxide, stabilized by 
hydrogen ion, and colloidal stannic oxide, stabilized by hydroxyl 
ion, was attributed to chemical neutralization. This view was 
supported by the observation that mutual precipitation was 
effected over a limited range of purity of sols, when the hydro- 
chloric acid and sodium hydroxide concentrations in the sols 
were approximately equivalent. The variation from equivalence 
was quite marked in case the sols were fairly pure. Thus a 
silica sol containing 16 SiO, to 1 NaOH was precipitated at 
various dilutions with a sol containing 13 Fe.O; to 1 FeCl. At 
the highest dilution possible for obtaining accurate data, mutual 
precipitation was observed when an amount of colloidal silica 
was added corresponding to but 50 per cent of the hydrochloric 
acid. This variation was attributed to the metastability of 
pure sols, which causes them to precipitate with a subnormal 
disturbance. This does not seem quite convincing since, in the 
absence of contamination other than that mentioned, the purity 
of the sols would scarcely be great enough to make them abnor- 
mally sensitive. Erratic results were also obtained when the 
amount of peptizing agent was too large, say three times as much 
as in the case referred to above. Thus, to obtain data to support 
a purely chemical mechanism involving the stabilizing agents, 
1 GaLEecKI and Kastovski: Kolloid-Z., 18, 143 (1913). 


? Kolloid-Z., 28, 258 (1921); 29, 16 (1921). 
* J. Am. Chem. Soc., 45, 2532 (1923). 


64 THE HYDROUS OXIDES 


it seems necessary to choose the experimental conditions to fit 
the case. While everyone will agree that the peptizing agents of 
two sols may interact under certain conditions, thus affecting 
the stability of each, such an interpretation of the mechanism of 
the mutual precipitation process would not account for the 
repeated observation of mutual precipitation of sols where inter- 
action between the peptizing agents is impossible or improbable. 
A sol peptized by hydrogen ion will be precipitated by a base or 
by a salt of a weak acid. From such observations alone, we 
might conclude that the precipitation of the sol was a result of 
chemical neutralization of the stabilizing agent. But the same 
sol will, in general, be precipitated with a small amount of an 
acid having a multivalent ion where chemical neutralization of 
hydrogen ion is impossible. 

If there are no disturbing influences, such as interaction between 
peptizing agents, Wintgen and Lowenthal! found the reciprocal 
precipitation of oppositely charged sols to be a maximum when 
the concentrations of the sols, expressed in ‘‘equivalent aggre- 
gates,’ are the same; that is, when equal numbers of charges of 
opposite sign are mixed. This rule does not hold in certain 
cases where a highly dispersed sol of one sign is mixed with a 
coarser sol of opposite sign, possibly because the smaller particles 
penetrate the larger ones and are precipitated by the electrolyte 
contained in the latter. 

Billitzer? found that gelatin in acid or neutral solution is a 
positive sol and so precipitates negative sols, but not positive 
ones such as hydrous ferric oxide; whereas gelatin in ammoniacal 
solution is a negative sol and precipitates hydrous ferric oxide. 
No precipitate is thrown down, however, if gelatin is first added 
to colloidal ferric oxide, followed by the addition of ammonia. 
In the latter case, we get a stable mixture of positive sols changed 
simultaneously to a stable mixture of negative sols by the addi- 
tion of hydroxy] ions. 

Brossa and Freundlich? studied the precipitation and repeptiza- 
tion of colloidal albumin by means of colloidal ferric oxide in 
the presence of electrolytes. The amount of albumin thrown 

1Z. physik. Chem., 109, 391 (1924). 


2Z. physik. Chem., 51, 148 (1905). 
3 Z. phystk. Chem., 89, 306 (1915). 


THE HYDROUS OXIDES OF IRON 65 


down by the ferric oxide sol decreases with decreasing concen- 
tration of electrolytes until eventually only a slight turbidity 
results, which disappears on adding a sufficient amount of ferric | 
oxide sol. Obviously, the colloidal ferric oxide adsorbs, and so 
keeps the colloidal albumin in solution. The ferric oxide-albu- 
min sol formed in this way is positively charged but is much 
more sensitive than the original sol. The sensitivity is at its 
maximum when the ferric oxide has adsorbed all the negative 
albumin sol that it can hold, without precipitation taking place. 
With increasing concentrations of ferric oxide sol, the sensitivity 
falls off, approaching that of the pure positive sol. If instead of 
adding an electrolyte to a ferric oxide-albumin sol, an albumin 
sol containing an electrolyte is precipitated with ferric oxide sol, 
the relationships are identical in many respects, particularly in 
the amount of albumin adsorbed by the ferric oxide. The 
presence of non-electrolytes such as urethane, camphor, and 
thymol have likewise been shown to increase the sensitivity of 
ferric oxide sol toward electrolytes.!. Freundlich attributes this 
to a lowering of the surface charge on the particles as a result of 
adsorption of a substance having a lower dielectric constant than 
water; but Michaelis? failed to detect any adsorption of non- 
electrolytes by hydrous ferric oxide or any effect of their presence 
on the adsorption of electrolytes. This failure to confirm Freund- 
lich’s hypothesis may be due to the limitations of the experimen- 
tal method in the systems investigated. The author* has 
observed a marked antagonistic action of phenol and isoamyl 
alcohol on the adsorption of barium ion by colloidal arsenious 
sulfide. 

Ferric Oxide Jellies —Although hydrous ferric oxide is usually 
thrown down as a gelatinous precipitate, jellies may be pre- 
pared by coagulation of a sol under suitable conditions. Thus, 
Grimaux‘ added to an excess of water an alcoholic solution of 
ferric ethylate which hydrolyzed very rapidly, forming a col- 
loidal ferric oxide. The sol was similar to Graham’s, but the 


1 FREUNDLICH and Rona: Biochem. Z., 81, 87 (1915); cf. Matsuno: 
Biochem. Z., 150, 139 (1924). 

2 MIcHAELIS and Rona: Biochem. Z., 102, 268 (1920). 

8 WeIsER: J. Phys. Chem., 28, 1253 (1924). 

4 Compt. rend., 98, 105, 1434 (1884). 


66 THE HYDROUS OXIDES 


particles were probably much smaller on account of the more 
rapid rate of hydrolysis. The sol coagulated spontaneously on 
standing for some time at room temperature; and more rapidly 
on heating or by the addition of electrolytes such as carbonice, - 
sulfuric, and tartaric acids; the nitrate, chloride, and bromide of 
potassium; the chlorides of sodium and barium, etc. The 
coagulum formed in every case was a transparent jelly, provided 
the sol was not agitated during the process of coagulation. 
Even with quite dilute sols, the jelly was firm; but contraction 
took place, very slowly in the cold and more rapidly at high 
temperature. 

With colloidal ferric oxide as with a number of other sols, 
slow uniform precipitation throughout the entire solution pro- 
duces a jelly, while rapid uneven precipitation results in contrac- 
tion and the consequent formation of a gelatinous precipitate. 
As compared with the usual Graham sol, Grimaux’s colloid is 
much more easily thrown down in the form of a jelly. This is 
accounted for by the fact that a sol formed by rapid hydrolysis 
in the cold will contain finer and more hydrous particles than 
one formed by prolonged dialysis in the cold or shorter dialysis 
in the hot. For the same reason, the coagulum from the Graham 
sol is much more hydrous and bulky than that obtained from a 
Péan de St. Gilles sol. The usual Graham sol can be precipi- 
tated as a jelly, provided the concentration is sufficiently high. 
Schalek and Szegvary? added electrolytes in amounts below their 
precipitation values to colloidal solutions containing 6 to 10 
per cent of ferric oxide and allowed the sols to stand quietly. 
After a time, the mixture set to a jelly that was no more cloudy 
than the original sol. This jelly solidified slowly after shaking 
up. The logarithm of the time required for solidification after 
shaking was found to be inversely proportional to the tempera- 
ture and to the concentration of coagulating electrolyte. Ultra- 
microscopic observation of the liquefaction process showed no 
change in the average distance between the particles and no 
formation of secondary particles. I am inclined to attribute the 
reversible sol-gel transformation in such a system to the breaking 

1 Cf. WaaneER: Kolloid-Z., 14, 150 (1914). 

2 Kolloid-Z., 32, 318 (1923); 33, 326 (1923); FREUNDLICH and ROSENTHAL: 
Ibid., 37, 129 (1925). 


THE HYDROUS OXIDES OF IRON 67 


up and subsequent realignment of the orienting forces among 
the particles. 

Ferric oxide jellies may be prepared also, by slow removal of 
the peptizing agent by dialysis. Thus Grimaux! obtained a firm 
jelly by dialysis of a negative sol prepared by peptization of the 
hydrous oxide with alkali in the presence of glycerin. If ammonia 
were used instead of alkali, and the sol exposed to the air, the 
slow loss of peptizing agent by evaporation resulted in the precip- 
itation of a jelly. Grimaux’s observations were confirmed by 
Fischer,? who prepared a firm jelly on prolonged dialysis of a sol 
containing but 1 per cent of iron. Unlike the more concentrated 
jellies of Schalek and Szegvary, this preparation broke down 
into a gelatinous precipitate when it was warmed, stirred, or 
frozen, Browne® obtained a jelly simply by allowing part of the 
water to evaporate Bey from a concentrated Graham sol of 
high purity. 


ADSORPTION BY HYDROUS FERRIC OXIDE 


Hydrous ferrric oxide as a technical adsorbent finds its most 
important use as a mordant in the dye industry and in the 
purification of municipal water supplies. These applications are 
considered in Chaps. XVI and XVII, respectively. 

Adsorption of Arsenious Acid.—Ninety years ago Bunsen‘ 
made the important discovery that freshly precipitated hydrous 
ferric oxide is an antidote for arsenic poisoning. As might be 
expected, this action was attributed by Bunsen to stoichiometric 
chemical union of ferric oxide and arsenious acid. While some 
people’ still maintain that iron arsenites of varying degrees of 
complexity® are formed when hydrous ferric oxide and arsenious 
acid are brought together under varying conditions, the investiga- 
tions of Biltz’ show the apparent interaction to be an adsorption 


1Compt. rend., 98, 1485 (1884). 

2 Biochem. Z., 27, 223 (1910). 

3 Private communication. 

4BunseEN and Berruorp: ‘‘Hydrated Ferric Oxide, an Antidote for 
Arsenious Acid,’’ G6ttingen (1834); cf. GurmpourtT: Arch. Pharm., (2) 28, 
69 (1840). 

5 REYCHLER: J. chim. phys., 7, 362 (1909); 8, 10 (1910). 

6 Oryna: Kolloid-Z., 22, 149 (1918). 

7 Ber., 37, 3138 (1904); cf. Kolloid-Z., 26, 179 (1920). 


63 THE HYDROUS OXIDES 


process in which the arsenic content of the hydrous oxide varies 
continuously with the concentration of arsenious acid in contact 
with it, giving a typical adsorption isotherm without a break or 
an evidence of discontinuity. 

Lockemann and Paucke! made a quantitative study of the 
adsorption of arsenious acid by charcoal, aluminum oxide, ferric 
oxide, and albumin. With ferric oxide, they find most complete 
adsorption when the iron is precipitated with stoichiometric 
quantities of ammonia; excess of ammonia or precipitation by 
potassium or sodium hydroxide decreases the adsorbability. 
This accords with Bradfield’s? observation that the most finely 
divided and most readily peptized particles of hydrous ferric 
oxide are formed by precipitation with but a very slight excess 
of ammonia. ‘The amount of hydrous oxide necessary to adsorb 
a given amount of arsenic can be calculated by means of the 
formula H = BAp where HE = milligrams ferric oxide, A = 
milligrams arsenic, and 6 and p are constants which vary with 
the temperature;’ but it should be pointed out that this equation — 
serves only as a simple approximation to the course of the 
adsorption.‘ 

Fischer and Juznitzky® injected colloidal ferric oxide simultane- 
ously with arsenious acid, under the skins of mice, and obtained 
partial protection from a fatal dose of arsenic. The negative 
colloidal hydrous oxide formed by peptizing ferric oxide with 
dilute alkali and glycerin was more effective than a positive 
Graham sol. Since it was thought improbable that a negative 
colloid would adsorb a negative ion, Fischer advanced the 
more improbable hypothesis that the antidotal effect was due to 
the formation of an iron-arsenic complex of some sort. These 
observations should be confirmed and a plausible explanation 
formulated. 

Catalytic Action.—Slightly hydrous or anhydous ferric oxide 
seems to have a relatively high adsorption capacity for gases 


1 Kolloid-Z., 8, 273 (1911). 

2 J. Am. Chem. Soc., 44, 965 (1922). 

3 Cf. LockKEMANN and Lucius: Z. physik. Chem., 83, 735 (1913). 

4 BoswELL and Dickson: J. Am. Chem. Soc., 40, 1793 (1918); cf. Mpcx- 
LENBURG: Z. physik. Chem., 83, 609 (1918). 

5 Biochem. Z., 27, 311 (1911). 


THE HYDROUS OXIDES OF IRON 69 


even at elevated temperatures since it is used as a catalyst in 
such industrial operations as the burning of hydrogen sulfide in 
the Chance-Claus process for recovering sulfur from alkali waste ;! 
in the Hargreaves and Robinson process for making salt cake;? 
and in the manufacture of sulfuric acid by the contact process.* 
In the latter process the conversion of sulfur dioxide to trioxide 
is 98 per cent using platinum as catalyst at 425°, dropping to 91 
per cent at 500° owing to dissociation of: the trioxide. The 
velocity with which sulfur dioxide and oxygen combine is less in 
the presence of ferric oxide so that it is necessary to work at a 
higher temperature when this catalyst is employed. On this 
account, the efficiency does not rise much over 60 per cent. It 
ought to be possible to make a ferric oxide catalyst that would 
work at as low a temperature as platinum if adsorption were the 
sole criterion of catalytic efficiency. Unfortunately this does 
not appear to be the case, as evidenced by such cases as charcoal 
which has a high adsorptive capacity but relatively poor catalytic 
properties. <A good catalyst is a good adsorbent but the converse 
is not necessarily true. 

Adsorption during Precipitation of Sol. ‘‘Acclimatization.’’— 
When a positive colloidal solution of hydrous ferric oxide is 
agglomerated by the addition of electrolytes, there is considerable 
adsorption of the negative precipitating ion.4 From precipita- 
tion and adsorption experiments, the order of precipitating power 
of the anions, deduced on the assumption that the most readily 
adsorbed anion precipitates at lowest concentration, was found 
to be different from the order obtained directly from adsorption 
data. The explanation of the discrepancy is that the adsorption 
measured is not ion adsorption only, but is ion adsorption plus 
adsorption of neutral salt during agglomeration. On account of 


1THorPE: ‘Dictionary of Applied Chemistry,” 5, 294 (1917); JoBLING: 
“Catalysis and Industrial Applications,” 33, (1916); RipzaL and TayLor: 
“Catalysis in Theory and Practice,” 112 (1919). 

2THoRPE: Loc. cit., 5, 25 (1913); Jopiina: Loc. cit., 32, (1915); RipEAL 
and Taytor: Loc. cit., 89 (1919). 

3 LuNGE and RetnwarptT: Z. angew. Chem., 17, 1041 (1904); KEPPELER, 
D’Ans, SUNDELL, and Karser: [bid., 21, 532, 577 (1908); KeppEter and 
D’Ans: Z. physik. Chem., 62, 89 (1908); W6HLER, PLUDDEMANN, and 
Wouter: [bid., 62, 653 (1908). 

4 Weiser and Mippieton: J. Phys. Chem., 24, 30 (1920). 


70 THE HYDROUS OXIDES 


the variation of the latter with the nature of the salt, the true 
order of adsorbability of the ion is masked. 

Since adsorption by neutralized colloidal particles during 
agglomeration is not negligible in any case and may rise to large 
proportions,! it is not surprising that such colloids as hydrous 
ferric oxide,” arsenious sulfide,” and albumin? require less electro- 
lyte to cause precipitation when added all at once than when 
added stepwise through a long interval of time, particularly when 
the slow addition produces fractional precipitation of the sol. 
This phenomenon is known as ‘‘acclimatization,”’ the connota- 
tion being that the colloid becomes acclimatized to its surround- 
ing when the electrolyte is added slowly and so more is required to 
produce a given result. It would appear, however, that the 
necessity for using more electrolyte to effect complete precipi- 
tation on slow addition arises not so much from the adaptability 
of the colloid to the presence of electrolytes, as from fractional 
precipitation, which not only removes ions owing to adsorption 
by neutralized particles but alters the stability of the sol by 
decreasing its concentration. From this point of view, the fac- 
tors which determine the excess required for a given slow rate of 
addition are: the extent to which the colloid undergoes fractional 
precipitation; the adsorbing power of the precipitated colloid; 
the adsorption of the precipitating ions; and the effect of dilution 
of the sol on the precipitation concentration of electrolytes.* 


THE COLOR OF HYDROUS FERRIC OXIDE 


That hydrous ferric oxide exists in many different colors, 
varying from yellow to violet red, is evident from the colors of 
the minerals. Thus, anhydrous hematite is black when ecrystal- 
line and red when powdered; turgite is a deep brown; limonite 
varies from a light brown to yellow; and limnite is a full yellow.° 
It is now known that hematite was the red ceramic pigment 
used by the prehistoric Indians of the Southwest. Their black 


1 FREUNDLICH: Z. physik. Chem., 44, 151 (1903); Weiser: J. Phys. 
Chem., 25, 405 (1921); 30, 22 (1926). 

> FREUNDLICH: Z. physik. Chem., 44, 143 (1908). 

’ H6BER and Gorpon: Beitr. chem. Physiol. Path., 5, 436 (1904). 

4 Weiser: J. Phys. Chem., 25, 413 (1921); 30, 20 (1926). 

6 Dammer: “ Handbuch anorg. Chem.,” 8, 304 (1893). 


THE HYDROUS OXIDES OF IRON FA 


pigment was the magnetic oxide of iron commonly called 
magnetite.! 

The variations in color of the anhydrous oxide appear to be due 
to the size of the particles. Thus Andersen? found plates of 
hematite as thin as 0.1u to be yellow by transmitted light, the 
color varying with increasing thickness through reddish brown to 
deep brown red or blood red; similarly, Wohler and Condrea? 
prepared anhydrous oxides that vary in color from yellow to red 
by simply varying the size of the particles, the red being the 
largest. Keane‘ attributes the yellow color of the so-called Mars 
pigments to finely divided ferric oxide which is kept from agglom- 
erating by the presence of aluminum oxide; and the yellow color 
which iron imparts to bricks, to sufficiently finely divided anhy- 
drous ferric oxide; when the particles are too large, the color is 
red rather than yellow.> Mott® obtained anhydrous red and 
yellow ferric oxide by volatilization in the electric arc; the yellow 
particles were the smaller. 

The hydrous oxide can be prepared in a variety of colors so 
similar to those of the anhydrous oxide that it seems reasonable 
to attribute the difference in color to the same cause—a differ- 
ence in the size of the hydrous particles. As a matter of fact, 
the variation in color from brown through yellow to red was 
shown by Malfitano and by Fischer to be associated with an 
increase in the size of the particles although they did not recog- 
nize the possible connection between the two. That there is a 
definite connection between particle size and color was shown by 
a series of experiments’ on the hydrolysis of ferric chloride solu- 
tions. The very finely divided brown particles may be trans- 
formed either into the larger yellow or the still larger brick red, 
by heating under suitable conditions. 

Since a very dilute solution of ferric chloride is colorless at the 
outset, changing spontaneously to yellow and then to reddish 


1 GERMANN: Science, 30, 20 (1926). 

2 Am. J. Sci., (4) 40, 370 (1913). 

3 Z. angew. Chem., 21, 481 (1908). 

4 J. Phys. Chem., 20, 734 (1916). 

‘Cf. ScurEtz: J. Phys. Chem., 21, 576 (1917); Yor: Ibid., 25, 196 (1921). 
6 Trans. Am. Electrochem. Soc., 34, 292 (1918). 

7 J. Phys. Chem., 28, 313 (1920), 


72 THE HYDROUS OXIDES 


brown,! it would appear that yellow hydrous particles are smaller 
than brown. This conclusion is unwarranted, since the color of a 
dilute colloidal solution is not necessarily determined by the 
color of the particles. Thus colloidal solutions of gold have 
been obtained which are red, violet, or blue by transmitted light ;? 
but this does not tell us the color of light reflected from the 
particles in the respective sols. Asa matter of fact, massive gold 
reflects yellow when compact and brown to black when porous. 
Small particles of gold which do not resonate are yellow to brown 
by reflected light and transmit blue. The surface color of gold 
is red by multiple reflection and very thin films are green by 
transmitted light.? Colloidal solutions with very fine particles 
of gold reflect green and transmit red. Hence, we conclude that 
the particles in the blue sol are yellow to brown, and in the red 
sol they are green.4 A deep-red Graham colloid from which can 
be thrown down a red-brown gelatinous precipitate appears 
distinctly yellow when diluted sufficiently. A 5-year-old brick- 
red Péan de St. Gilles sol appears yellower on dilution, although 
the reddish color persists. It is possible that the reddish-brown 
particles in a red Graham colloid transmit more yellow than red 
when sufficiently highly dispersed. At any rate, there seems no 
reason for believing the yellow colloid formed by hydrolysis of. 
ferric chloride to be other than a highly diluted Graham sol. 
The color of such a solution becomes redder with age, owing to 
the formation of more red-brown colloidal hydrous oxide. A 
thousandth normal solution which Goodwin found to be com- 
pletely hydrolyzed in a few hours, is very much redder than a 
fiftieth normal or hundredth normal solution after 24 hours. 

It appears that a colloidal solution of hydrous ferric oxide 
contains varying amounts of small highly hydrous red-brown 
particles and larger less hydrous yellowish-brown particles, both 
of which may be converted into still larger and less hydrous 
brick-red particles by heating at 100°. If the conditions are such 
that the red particles remain in colloidal solution, we have the 


1AnTONY and Giauio: Gazz. chim. ital., 25, 1 (1895); Goopwin: Z. 
physik. Chem., 21, 1 (1896); cf. Waaner: Kolloid-Z., 14, 150 (1914). 

2? FaraDay: Phil. Trans., 147, 145 (1857). 

3’ BemtBy: Proc. Roy. Soc., 72, 226 (1913). 

4Cf. Bancrort; ‘Applied Colloid Chemistry,’ 204 (1921). 


THE HYDROUS OXIDES OF IRON 73 


brick-red Péan de St. Gilles colloid. Bradfield! demonstrated 
conclusively that the reddish-brown precipitate formed by 
adding ammonia to ferric chloride solution until minute floc- 
cules are barely visible, contains both very small highly hydrous 
dark-brown particles and larger less hydrous yellowish-brown 
particles which can be separated rather sharply from each other 
by centrifuging the suspended precipitate. Both the reddish 
and yellowish particles in a sol formed by heating a 1 per cent 
solution of ferric chloride from room temperature to the boiling 
point appear to be transformed to larger less hydrous bright- 
red particles by heating at 100°. The granular ocher-yellow 
particles formed by heating a more concentrated solution slowly 
are not converted into the red at this temperature. This differ- 
ence might be ascribed to the dense granular character of the 
particles which precipitate on heating the more concentrated 
solutions; but it will be recalled that a yellow Péan de St. Gilles 
colloid formed by slow hydrolysis of ferric acetate is not changed 
to red by prolonged boiling of the sol. The yellow particles 
formed under certain conditions lose water much less readily at 
100° than the reddish brown; and this seems to account for the 
difference in behavior. As previously pointed out, Keane and 
Scheetz have shown the yellow color of bricks to be due to finely 
divided anhydrous ferric oxide which is kept from agglomerating 
by alumina and probably by certain other substances as well. 
This requires a rather high percentage of alumina. In the 
so-called Mars pigments which are yellow, the ferric oxide is in the 
hydrous state; and in this condition it agglomerates less readily 
to the red oxide, and less alumina is required to prevent the 
transformation. Since the yellow oxide retains its water more 
tenaciously than the brown, it is natural to inquire into the cause 
of the increased stability. In view of the synthesis of a yellow 
monohydrate of ferric oxide by the slow hydrolysis of ferric sul- 
fate,” it would appear reasonable to conclude that the yellow oxide 
which does not lose water and become red at 100° is ferric oxide 
monohydrate. The yellow oxide that apparently loses water and 
agglomerates to red at 100° may be regarded as hydrous ferric 
oxide in which the particles are somewhat ee and less hydrous 


1 J. Am. Chem. Soc., 44, 965 (1922). 
2 PosNJAK and Nearer: J. Am. Chem. Soc., 44, 1965 (1922). 


74 THE HYDROUS OXIDES 


than the brown. But as I am aware of no case in which yellow 
hydrous particles free from brown appear to be transformed into 
red by heating at 100°, it is open to anyone to assume that the 
yellow particles are really never transformed into red; but that 
the bright-red color formed by agglomeration of the brown oxide 
masks the yellow monohydrate. 

If one objects to the assumption that the yellow colloid is a 
hydrous monohydrate, another alternative is to attribute its 
stability at 100° to adsorption of some salt. Bancroft! suggested 
that the yellow color of the oxide is due to the presence of 
adsorbed ferric salt. This suggestion was based on Fischer’s 
observation that the brown colloid goes over into red in the pres- 
ence of hydrochloric acid; on Malfitano’s experiment, that the 
brown colloid is transformed into the yellow by boiling with 
ferric chloride; and on Phillips’ method of preparing the yellow 
oxide by oxidation of ferrous carbonate. Malfitano’s observa- 
tion is inconclusive, since boiling a ferric chloride solution alone 
will give a yellow colloid. Moreover, the author precipitated 
the hydrous oxide in a gelatinous form in the presence of a large 
excess of ferric chloride, a condition favorable to adsorption of 
ferric salt; and yet the oxide was distinctly red. Hence, there 
seems no reason for attributing the color of the yellow colloid 
to adsorbed iron salt. Bancroft’s hypothesis was the outgrowth 
of the observation that the yellow colloid is formed when the 
adsorption of an iron salt is a possibility. The converse appears 
not to be the case, namely, that the adsorption of an iron salt 
always results in the formation of a yellow hydrous oxide. 
Although the adsorption of an iron salt does not impart a yellow 
color to a hydrous ferric oxide, it is possible that the yellow oxide 
which is not converted to red by heating at 100° is stabilized by 
adsorbed iron salt. 


LOWER OXIDES OF IRON 


Hydrous Ferrous Oxide.—On account of its relatively low 
solubility, hydrous ferrous oxide comes down in a highly gela- 
tinous form when a solution of ferrous salt is treated with potas- 
sium or sodium hydroxide.? The gel is white when absolutely 


1 J. Phys. Chem., 19, 282 (1916). 
2Scumipt: Liebig’s Ann. Chem., 36, 101 (1840). 


THE HYDROUS OXIDES OF IRON 75 


pure; but owing to the difficulty in excluding all air during pre- 
cipitating and washing, it is usually obtained as a green hydrous 
mass. Even when dried, the gel oxidizes so readily in the air 
that the whole mass sometimes becomes incandescent. 

As ordinarily prepared, the gel is hydrous FeO; but de Schulten! 
obtained the monohydrate or hydroxide by crystallization from 
solution in strong caustic soda. The crystals were small green 
prisms which oxidized very rapidly in the air even after they 
were washed with alcohol and ether, and dried in hydrogen. 
Owing to its strong affinity for oxygen, the oxide is a powerful 
reducing agent, converting nitrites and nitrates to ammonia, a 
reaction that may be used for the quantitative estimation of 
the substances.’ 

Whitman, Russell, and Davis? find that the rate of corrosion of 
iron in salt solutions parallels the solubility of ferrous hydroxide 
in these solutions. It is suggested that this is due to changes in 
film protectivity with the solubility of the ferrous salt. 

Hydrous Ferro-ferric Oxide.—The gel of ferro-ferric oxide is 
obtained by adding alkali to a solution containing equivalent 
amounts of ferrous and ferric salts. If washed and dried out of 
contact with air, it is a magnetic brownish-black mass containing 
an indefinite amount of water.‘ 


1 Compt. rend., 109, 266 (1889). 

2 MIYAMOTO: v2 One Soc. Japan, 48, 397 (1922). 

3 J. Am. Chem. Soc., 47, 70 (1925); cf. Frrmnp: J. Chem. Soc., 119, 932 
(1921). 

4 WountER: Liebig’s Ann. Chem., 22, 56 (1838); Lerort: Compt. rend., 
69, 179 (1869). 


CHAPTER III 
HYDROUS CHROMIC OXIDE 


Composition.—The addition of ammonia or an alkali to a 
solution of chromic salt precipitates chromic oxide as a highly 
hydrous gel, the composition and properties of which depend on 
the conditions of precipitation and the subsequent treatment. 
The gel is frequently designated chromic hydroxide and assigned 
the formula Cr (OH); or Cr.03:3H.O, although 35 years ago van 
Bemmelen! determined the isotherm for chromic oxide and water 
between 15 and 280° and found no evidence of any definite 
hydrate. As van Bemmelen’s observations have been confirmed 
by von Baikow,? it is altogether likely that the various so-called 
hydrates described from time to time* were merely hydrous 
chromic oxides dried to a composition expressible by a Dalton 
formula. 

Férée* claims to have obtained the compound Cr.0;: HO by 
electrolysis of a neutral solution of chromium chloride with a 
platinum cathode. The brownish-black amorphous powder 
loses water on heating to 80°; but it is questionable whether 
this is a definite inversion temperature at which all the water is 
lost. It is also claimed by some that a green hydrate, Guignet’s 
green,’ is formed by fusing 1 part of bichromate of sodium, potas- 
sium, or ammonium with 3 parts of boric acid; but there is a 


1 Rec. trav. chim., 7, 37 (1888). 

2 J. Russ. Phys.-Chem. Soc., 39, 660 (1907). 

3 ScHAFFNER: Liebig’s Ann. Chem., 61, 169 (1844); Stmwert: Jahresber., 
242 (1861); Lomweu: J. Pharm., (8) 7, 328, 401, 424 (1845); Fremy: 
Compt. rend., 27, 269; 30, 415 (1847); 47, 883 (1858); LErortT: J. Pharm., 
(3) 18, 27 (1850); Vincent: Phil. Mag., (4) 18, 191 (1850). 

4 Bull. soc. chim., (3) 25, 620 (1901); cf. Bunsmn: Pogg. Ann., 91, 619 
(1854); GeuruEeR: Liebig’s Ann. Chem., 118, 66 (1861). 

5 GUIGNET; Jahresber., 761 (1859). 

76 


HYDROUS CHROMIC OXIDE 77 


difference of opinion as to the formula.! Wohler and Becker? 
obtained a similar green pigment by heating the ordinary 
oxide in an autoclave at 180 to 250°. It retains its color when 
dried at 80° but darkens gradually and loses water above this 
temperature. The oxide was taken to be a definite hydrate, 
since its composition on drying at 80° may be represented by 
the formula 2Cr,0;-38H,O. The green pigment prepared in 
any way is amorphous in character and, like the ordinary pre- 
cipitated oxide, loses water continuously as the temperature is 
raised. Of course, it is entirely possible to dry the pigment 
under such conditions that the percentage composition may be 
expressed by a simple formula, but that does not prove that a 
true hydrate is formed. 

Ageing.—Hydrous chromic oxide, freshly precipitated from 
a cold chromic salt solution with an alkali or ammonia, is readily 
soluble in acids giving the corresponding salts and is peptized by 
alkali hydroxides with the formation of a colloidal solution. On 
standing, the oxide undergoes a change in physical character 
accompanied by a marked decrease in solubility and reactivity. 
This process called ‘‘ageing” is probably due to the growth and 
agglomeration of primary colloidal particles, since the velocity of 
change increases rapidly with rising temperature and is hastened 
in a medium possessing a slight solvent action. Recoura? 
followed the change by determining the molar heat of solution in 
hydrochloric acid of the oxide precipitated with acid from the 
colloidal solution in alkali, after definite intervals of time. From 
his results given in Table III, it will be noted that the change in 
the heat of solution is quite marked during the first few minutes. 
This change is accompanied by a similar decrease in solubility. 
Since the ageing is more rapid at higher temperatures, the oxide 
precipitated at 100° is much less.soluble than that thrown down 
at room temperature. 


1S8atveTar: Compt. rend., 48, 295 (1859); ScunuRER-Kestner: Dinglers 
polytech. J., 176, 386 (1865); EnneR and Hus: Farbezig., 15, 2106, 2157, 
2213, 2268, 2319 (1910). 

2Z. angew. Chem., 21, 1600 (1908); 24, 484 (1911). 

3 Compt. rend., 120, 1335 (1895); cf. FricKkm and WINDHAUSEN: Z. physik. 
Chem., 118, 248 (1924); Z. anorg. Chem., 1382, 273 (1924), 


78 THE HYDROUS OXIDES 


TaBLE II].—Mouar Heat or SoLutTion or Hyprous CHROMIC OXIDES 
PRECIPITATED FROM SOLUTION IN ALKALI 














: Molar heat of : Molar heat of 
Time i 5 Time ‘pee : 
solution, calories solution, calories 
0 20.70 7 hours 2.40 
10 minutes 7.90 1 day 1,75 
1 hour 5.80 7 days . 1.20 
2 hours 3.90 30 days 0:75 
4 hours 2.85 60 days 0.50 


Solutions of hydrous chromic oxide in alkali were found by 
Bourion and Senechal! to lose their reducing power toward 
hydrogen peroxide on standing. The reaction (loss of reducing 
power) with a solution containing 0.938 gram chromic oxide and 
58 grams sodium hydroxide per liter appeared to be approximately 
tetramolecular for the first 8 hours. The results were attributed 
to the transformation of the original oxide into complexes of 
decreasing chemical activity, the tetramolecular order being 
only apparent. Bourion and Senechal evidently believe that 
hydrous chromic oxide dissolves in alkali with the formation of 
chromite; but in reality it is held in colloidal solution, for the 
most part. The decreased activity on standing is due to a 
gradual change in the physical character of the particles, a 
change that is sufficiently marked with a concentrated sol to 
cause partial precipitation in a short time. This transformation 
from a very soluble to a less soluble and less reactive form of 
hydrous chromic oxide has very naturally been attributed to 
the existence of definite allotropic or isomeric modifications. 
This is very unlikely, particularly since there is no inversion 
point for a soluble and an insoluble modification. Between 
these two extremes of solubility, it is possible to prepare an 
indefinite number of hydrous oxides, each differing slightly from 
the others in water content, in size of particles, in structure of 
the mass, and consequently, in reactivity with acids and alkalies.” 

The Glow Phenomenon.—When hydrous chromic oxide is 
heated at a suitable rate to temperatures around 500°, it evolves 

1 Compt. rend., 168, 59, 89 (1919). 

2 FRICKE and WINDHAUSEN: Z. anorg. Chem., 132, 273 (1924). 


HYDROUS CHROMIC OXIDE 79 


enough heat to cause it to become incandescent. The tempera- 
ture at which the glowing takes place varies with the sample 
and with the method of heating. Berzelius, Wohler,! and Endell 
and Rieke? give approximately 500° for the glow temperature; 
Le Chatelier* gives 900°, and Rothaug? finds it to vary between 
420 and 680°, depending on whether the precipitate is in a pow- 
dery or granular form. The glow is regarded by some® as an 
accompaniment of the transformation of one allotropic modifica- 
tion of the oxide to another; but this seems unlikely, 
since the glowing depends on the rate of heating® and since the 
glow temperature varies with the size of the particles. Moreover, 
the phenomenon is observed with a number’ of hydrous oxides as 
well as other substances; and it is improbable that all of them 
should exist in two forms. Wohler found that the glowing 
is increased by all conditions which favor hydrosol formation in 
the preparation of the oxide, for example, the use of dilute solu- 
tions of reagents, the use of chloride rather than sulfate, and of 
potassium hydroxide rather than ammonium hydroxide. More- 
over, the glow was found to be greater, the greater the adsorption 
capacity of the precipitate, indicating that the phenomenon 
is connected closely with the surface area. Under the same 
conditions of heating, the heat evolved by 1 gram of oxide was 
sufficient to raise its temperature anywhere from 50 to 100°, 
depending altogether on the extent of surface. 

In the light of Wohler’s observations, there is little doubt but 
that the glow is due to a very sudden decrease in the large sur- 
face of the oxides prepared by precipitation. The oxides thrown 
down under different conditions vary in the size of the particles 
and the amount of enclosed water and hence in the extent of 
surface. The maximum glow and heat evolution are obtained 
when the sample, made up of finest particles, is heated rapidly 

1 Kolloid-Z., 11, 241 (1913). 

2 Zentr. Min. Geol., 246 (1914). 

3 Bull. soc. chim., (2) 47, 303 (1887). 

4Z. anorg. Chem., 84, 165 (1913). 

5 Moissan: Bull. soc. chim., (2) 34, 70 (1880); Ann. chim. phys., (5) 21, 
199 (1880); Le CuatenipR: Bull. soc. chim., (2) 47, 303 (1887); Mrxmr: 
Am. J. Sci., (4) 26, 125 (1908); 39, 295 (1915). 

6 Stmwert: Jahresber., 243 (1861); cf. Mixer: Loc. cit. 

7 Wouter: Kolloid-Z., 11, 241 (1913); Enpeui and Rieke: Loc. cit. 


80 THE HYDROUS OXIDES 


to the glow temperature, which is in the neighborhood of 500.° 
If a fine-grained precipitate is heated very slowly or kept for 
some time below the glow temperature, there is a gradual, 
instead of a sudden, diminution of surface, which is not accom- 
panied by incandescence. Thus, glowing at elevated tempera- 
tures is the visible manifestation of the coalescence of primary 
colloidal particles into larger masses, involving a marked decrease 
in specific surface. Similarly, at ordinary temperature the 
gradual change in solubility, in reactivity, and in molal heat of 
solution in hydrochloric acid is due to coalescence of the small 
primary particles into larger primary particles with the concomi- 
tant diminution in specific surface. This change is a truly irre- 
versible process differing from ordinary coagulation in which the 
primary particles merely form secondary aggregates with very 
little change in specific surface. 

Color.—Hydrous chromic oxide can be obtained in various 
shades from a clear gray blue to a dark green. Certain of these 
colors, such as chrome green and Guignet’s green, constitute the 
most permanent green pigments. The color of the oxide freshly 
precipitated in the cold is variously described by different people 
as bluish, violet blue, clear blue, clear gray blue, and gray violet. 
The shade differs somewhat, depending on whether it is precip- 
itated from a green or violet chromic salt. On drying the 
precipitate, the color changes to a distinct green, and the dry 
amorphous oxide is described as vivid green. Mention has 
been made of the transformation of the ordinary precipitated 
oxide into Guignet’s green by ageing in an autoclave at 180 to 
250°. The rate of precipitation seems to have a marked effect 
on the color. Thus, Casthelez and Leune? claim to have pre- 
pared an oxide with a richer and purer color than Guignet’s 
green, simply by slow precipitation at ordinary temperatures of a 
green solution of a chromic salt with aluminum hydroxide, zine 
carbonate, zinc sulfide, or zinc. This observation was confirmed? 
by adding mossy zine to a solution of green chromic chloride and 
allowing to stand at 25° for several days. ‘The clear dark-green 
oxide which formed was much more granular than the gray-blue 

1 See p. 57. 

2 Bull. soc. chim., (2) 10, 170 (1868). 

3 WEISER: J. Phys. Chem., 26, 410 (1922). 


HYDROUS CHROMIC OXIDE 81 


gelatinous oxide obtained by rapid precipitation; moreover, it 
was quite insoluble in normal sulfuric acid. 

Berzelius! believed the oxides precipitated from violet and 
green solutions to be isomers, since they redissolve in acids giving 
solutions with the original colors. This, however, seems to 
depend altogether on the method of procedure. Thus Recoura? 
added alkali to a green solution until a precipitate was formed 
which was dissolved at once in hydrochloric acid giving a violet 
solution; while the hydrous oxide precipitated from what Recoura 
claimed to be Cr2.OCl, gave a green solution. It would appear, 
therefore, that the hydrous oxides from different-colored solutions 
are the same in chemical structure, the individual variation in 
color and solubility arising from the difference in the physical 
character of the hydrous particles’ and the structure of the mass. 

The wide difference in color between the gray-blue precipitated 
oxide and Guignet’s green causes Wohler and Becker to regard 
the two substances as hydrate isomers bearing a relation to each 
other similar to the relationship between blue and green chromic 
chloride. In support of this view, they show that two prepara- 
tions with the same water content have a different vapor pressure; 
and that the ordinary oxide can be convered into Guignet’s 
green by heating in an autoclave. These evidences are alto- 
gether inconclusive. In the first place, the vapor pressure of a 
hydrous oxide is determined not only by the amount of water it 
contains but by its structure;* and since the conditions of forming 
Guignet’s green and the ordinary oxide are so different, it is not 
surprising to find variation in the size of the particles and the 
structure of the masses of each, as is evidenced not only by 
difference in vapor pressure but by difference in color. In the 
second place, Wohler and Becker were unable to find an inversion 
temperature of gray-blue oxide to Guignet’s green, and the fol- 
lowing experiments® indicate that a definite transition point 
does not exist: 20-cubic-centimeter portions of a solution con- 

1 “Lehrbuch,” 5th ed., 2, 315 (1848). 

2 Compt. rend., 104, 1227 (1887); Ann. chim. phys., (6) 10, 1 (1887); cf. 
Oute: Z. anorg. Chem., 52, 48 (1907). 

3 Cf., however, Recoura: Loc. cit.; LonwEeu: J. Pharm., (3) 7, 323, 401, 
424 (1845); Fremy: Ann. chim. phys., (3) 28, 388 (1848). 


4 Van BEMMELEN: ‘Die Absorption,” 239 et seq. (1910). 
6 Weiser: J. Phys. Chem., 26, 409 (1922). 


82 THE HYDROUS OXIDES 


taining 0.2 gram of chromium chloride were treated with just 
enough sodium hydroxide solution to cause complete precipita- 
tion at the various temperatures shown in Table IV; and the 
precipitates were kept at this temperature for a definite length 
of time. For temperatures above 100° the precipitations were 
carried out in an autoclave. The color varies continuously from 


TaBLE IV.—EFrrect oF TEMPERATURE OF PRECIPITATION ON THE COLOR 
oF Hyprovus CHRomMIC OxIDE 








EDL oes Time of heating Color of precipitate 
degrees 
0 30 minutes Gray blue 
50 30 minutes Greenish blue 

100 30 minutes Bluish green 

150 30 minutes Green with faint tinge of blue 

200 30 minutes Clear green 
200-225 15 hours Bright green 


gray glue to clear green with increasing temperature of precipita- 
tion. This indicates that the various colors are not due to iso- 
mers but to a difference in the size of the particles, the structure 
of the mass, and the amount of water enclosed under the different 
conditions of formation. As the color changes from blue to clear 
green with increasing temperature of precipitation, the oxide 
becomes less gelatinous, less soluble in acids, and less readily 
peptized by alkalies. 


CHROMIC OXIDE SOLS 


The Positive Sol Formed by Peptization Methods.—Graham! 
prepared colloidal hydrous chromic oxide by peptizing the 
freshly precipitated oxide with chromic chloride and dialyzing 
to remove excess electrolyte. The colloidal solution is dark 
green, and can be diluted with water or heated; but is very 
instable in the presence of salts. 

Neidle and Barab? investigated the dialysis of a colloidal 
solution prepared by the Graham method. The sol was placed 


21 Phil. Trans., 151, (1), 183 (1861). 
J. Am. Chem. Soc., 38, 1961 (1916). — 


HYDROUS CHROMIC OXIDE 83 


in a parchment membrane surrounded by water. In one series 
of experiments the water was changed at intervals; while in a 
second series, a continuous flow of water through the dialyzer 
was maintained. Colloidal particles diffused through the mem- 
brane in both cases. In the intermittent dialysis, the sol 
continued to diffuse until but little remained within the mem- 
brane; whereas in the continuous process, the passage of the sol 
ceased after a time, and 75 per cent remained within the mem- 
brane. The growth of the colloidal particles during dialysis was 
influenced by two factors: agglomeration following removal of 
peptizing agent, and growth of nuclei by hydrolysis of adsorbed 
chloride by adsorbed water. In the intermittent process, the 
removal of peptizing agent was not rapid enough to cause sufh- 
cient agglomeration to prevent the passage of the colloid through 
the particular membrane; while in the continuous process, a 
gradual growth of the particles resulted finally in their retention 
by the membrane. By continuous dialysis at a high tempera- 
ture,! the time required to get a colloidal solution containing a 
minimum amount of peptizing agent may be shortened by weeks. 

Bjerrum? obtained small amounts of basic chlorides having 
the formulas Cr(OH)Cl, and Cr(OH)2Cl on adding alkali to chro- 
mic chloride,* and Recoura‘ claimed to get CreOCl, by the oxida- 
tion of CrCl, in the air;> but it is unlikely that any quantity of 
basic salt is present in the well-dialyzed solution of hydrous 
chromic oxide in chromic chloride. Neidle and Barab dialyzed 
such a colloidal solution in the hot until the ratio, equivalents 
Cr: equivalents Cl, was above 1500. It seems absurd to regard 
such a solution as a basic salt; on the other hand, it does not pre- 
clude the possible presence of a trace of basic salt in a highly 
purified sol. For the most part, however, the sol consists of 
hydrous chromic oxide peptized by preferential ‘adsorption of 
chromium and hydrogen ions. 


1 J. Am. Chem. Soc., 39, 71 (1917). 

2Z. physik. Chem., 73, 724 (1910); cf. also DenHam: J. Chem. Soc., 93, 41 
(1908). 
3 Cf. Fiscnoer: Z. anorg. Chem., 40, 39 (1904). 

4 Ann. chim. phys., (6) 10, 1 (1887). 

5 See also Moserea: J. prakt. Chem., 29, 175 (1843); Lonwe.u: J. Pharm., 
4, 424 (1843); P&iicot: Compt. rend., 21, 24 (1845); Orpway: Am. J. Sci., 
(2) 26, 202 (1858); Ouie: Z. anorg. Chem., 52, 62 (1906). | 


84 THE HYDROUS OXIDES 


If it were possible to dialyze the sol until all the chromic 
chloride were hydrolyzed and practically all of the hydrogen 
were adsorbed either as hydrogen chloride or as hydrogen ion, 
the composition of the. sol might be represented by the general 
formula 


[(jaCr2O3-yHCl-2H.0) H’),-(n — q)CV] + gCl’ 


where qg represents the excess of adsorbed hydrogen ions over 
adsorbed chloride ion, that is, the charge on the colloidal particles. 
Actually, the solution as well as the sol particles will contain 
hydrogen ions and may contain chromium ions; and the particles 
may contain adsorbed chromium. If hydrochloric acid is placed 
on one side and a well-dialyzed sol on the other side of a mem- 
brane permeable to hydrogen and chloride ions but not to the 
colloidal particles holding an excess of adsorbed hydrogen ion, 
a Donnan equilibrium will be set up with the attending con- 
centration, osmotic, and electrical effects. Bjerrum? placed a 
chromic oxide sol in a collodion bag and surrounded it by solutions 
of hydrochloric acid of varying concentration. The outside solu- 
tion was renewed daily until equilibrium was established, and 
the osmotic pressure and membrane potential were measured in 
a special apparatus: Some observations are recorded in Table V. 
The concentration cz of HCl in the outer solution and the con- 
centration [Cr.03] of the sol are expressed in mols per liter. The 


TaBLeE V.—Osmortic PRESSURE AND MEMBRANE POTENTIAL OF A CHROMIC 
Ox1pE Sou aT 18°. Tue ‘ EQuivALENT AGGREGATE” OF THE SOL 

















HCI (ee ee | m = 1000 | m= 900 | m = 250 
x [Cr203] Pi Ey. |= oie TT Sa en 

Ae | VEN | Ae Ver | Ae Py; Ae 

0.010 0.042 THe 5 3 ne 0) is eal: 14 4.1 18 
0.010 0.038 6.0 6.4 12 0.9 4853 1.9 15 oe re 20 
0.005 0.038 9.7 10.5 14 0.9 14 1.9 15 3 yall ilg 
0.005 OF027 bape Olle ee 14 0.6 14 1.4 16 Oe Fi 20 
0.005 0.027 4.8 9.2 14 0.6 14 1.4 16 2.6 20 
0.005 0.026 4.4 9.1 14 0.6 15 1.3 16 2.5 21 
0.0025 0.026 iO 16.1 14 0.6 15 i123 16 DAES 18 
0.001 - 0.026. 17.8 28.2 14 0.6 14 L38 14 ANE 15 
0.005 0.025 4,2 q04 14 0.6 14 133 16 2.4 20 
0.010 0.025 20) 5.2 12 0.6 14 Les 18 2.4 100 

Of apr 


2Z. physik. Chem., 110, 656 (1924). 


HY DROUS CHROMIC OXIDE 85 


osmotic pressure P; is given in centimeters of water and the 
membrane potential H,, in millivolts. 

The measured osmotic pressured P is the sum of the pressure 
P;, of the colloidal particles and the pressure P2, caused by the 
difference in the number of dialyzed particles within and without 
the membrane, that is, 


P=P,+P, (1) 
According to Avogadro 


P, = rr. (C203) (2) 
m 
and according to Avogadro and Donnan, 
— pr. [Gr20s}? 
Reaalt lh sera as, (3) 


where RT is 24,700 at 18°; m is the number of Cr.03 molecules 
in a colloidal particle, and Ae the equivalent aggregate, that is, 
the number of Cr.O; molecules carrying one electrical charge.! 
P is determined directly for the different values of cz and [Cr2O3] 
as given in the table. Corresponding values of P; are calculated 
from (2) for various assumed values of m; and from these P; 
values, P.. values can be gotten from (1) and Ae values from 
(3). Bjerrum took the values of m which give the most constant 
values of Ae as the correct m; and the average value of Ae as 
the correct Ae. One would conclude from the table that m is 
greater than 250; but the true value is quite indefinite. Bjerrum 
says m is approximately 500 and Ae approximately 15;? in other 
words, the colloidal particle contains something like 1,000 chro- 
mium atoms and carries 30 free positive charges. 

From the osmotic-pressure measurements, Bjerrum also calcu- 
lates the amount of free chloride ion in the sol. Subtracting 
this from the total chloride concentration is said to give the 
adsorbed chloride ion. From such considerations, the conclu- 
sion is reached that the colloidal particle contains 1,000 chro- 
mium atoms, carrying a total of 240 positive charges, 210 of 
which are neutralized by adsorbed chloride ion. This is prob- 


10f. Zstiamonpy: ‘‘ Kolloidchemie,’’ 206 (1925). 
2 Cf., however, WINTGEN and LOWENTHAL: Z. physik. Chem., 109, 378 
(1924), 


86 THE HYDROUS OXIDES 


ably incorrect, as a part of the chlorine is doubtless adsorbed 
as chloride and not as ion. 

It is of interest to compare the value m = 500 for a Cr2Q; sol, 
aged by prolonged boiling, with m = 750,000 for an aged Fe.O; 
sol, as reported by Wintgen and Biltz.! I doubt very much 
whether there are 1500 times as many molecules in an aged iron 
sol as in an aged chromium sol. It is more likely that the limits 
of the experimental methods employed by both Bjerrum and 
Wintgen render the values for both sols of doubtful accuracy. 

Richards and Bonnet? digested hydrous chromic oxide with 
chromium sulfate on the steam bath for several hours, obtaining 
a green solution which appeared to them to be a basic salt, 
Cr(OH)SO,. <A violet solution shaken for several days with 
hydrous chromic oxide changed to green which had a composition 
that could be expressed by the formula Cr3(OH)7;(SO.)4. While 
these observations prove nothing one way or the other, they 
indicate that chromic sulfate solution peptizes rather than 
reacts with hydrous chromic oxide. Seymour-Jones’ reduced a 
solution of sodium bichromate with sulfur dioxide, obtaining a 
solution which dialyzed completely through collodion mem-. 
branes and passed unchanged through a hardened ultrafilter. — 
Such a solution should have a basicity equivalent to Cr(OH)SQO,, 
but according to Basset’ it contains a mixture of 95 to 96 per 
cent chromic sulfate and 4 to 5 per cent chromium dithionate. 
Hence, the existence of a basic salt of the formula Cr(OH)SO, ~ 
has not been established. If such a basic salt were present in 
Seymour-Jones’ solution, it was readily dialyzable, a circum- 
stance that would argue against the presence of any basic chloride, 
Cr(OH)Cl., in the well-dialyzed Graham sol. Werner? isolated 
a crystalline basic sulfate of the formula [Cr(OH)e-: (H20).4|2S0x,; 
but this was done in a special way. 

_ The nature of the basic solutions of chromic sulfate is of interest 
in connection with chrome tanning and chrome mordanting, which 
will be taken up in detail in Chaps. XV and XVI, respectively. 


1Z. physik. Chem., 107, 414 (1923). 
2 Z. physik. Chem., 47, 29 (1904). 

8 J. Ind. Eng. Chem., 15, 77 (19238). 
4 J. Chem. Soc., 83, 692 (1903). 

® Ber., 41, 3447 (1909). 


HYDROUS CHROMIC OXIDE 87 


Paal! prepared a colloidal solution of hydrous chromic oxide 
by reduction of a solution of ammonium chromate with colloidal 
platinum in the presence of the sodium salt of protalbinic acid 
which acts as a protective colloid. The preparation contained 
colloidal hydrous oxide, colloidal platinum, unchanged ammo- 
nium chromate, and sodium protalbinate. It may be purified 
to some extent by dialysis. | 

The Positive Sol Formed by Hydrolysis Methods.—If a solu- 
tion of a ferric or aluminum salt is boiled with sodium acetate, 
there is formed the acetate of the trivalent metal which hydro- 
lyzes, precipitating the respective hydrous oxides. By working 
at low concentrations, colloidal solutions of the hydrous oxides 
of iron and aluminum may be prepared by hydrolysis of the 
acetates;? but chromic acetate behaves differently.* Reinitzer* 
boiled solutions of chromic chloride and sulfate with sodium 
acetate for a short time, obtaining a violet solution but no pre- 
cipitate. This solution was not precipitated in the cold with 
sodium or potassium hydroxide, ammonia, ammonium hydro- 
sulfide, ammonium carbonate, sodium phosphate, barium hy- 
droxide, or barium carbonate; but was thrown down in the hot 
by all the above reagents except sodium phosphate. A similar 
solution which gave no test for chromic ion was obtained by 
allowing the solution of chromic salt and sodium acetate to stand 
in the cold for a sufficient length of time. A slow action in the 
cold in the presence of alkalies is evidenced by a change in color 
of the solution and the formation of a jelly on standing quietly. 

Although some colloidal hydrous chromic oxide may be formed 
by boiling a chromic salt solution with sodium acetate, it is 
altogether probable that this process results chiefly in the forma- 
tion of one or more of the complex chromic acetates, a number of 
which have been isolated in a definite crystalline form by Werner,° 
and by Weinland and his pupils.* Solutions of these salts do not 
give the usual reactions for chromic ion since the chromium is a 

1 Ber., 47, 2211 (1914). 

2 Cf. WEISER: J. Phys. Chem., 24, 277, 505 (1920). 

3 ScuirF: Liebig’s Ann. Chem., 124, 168 (1862). 

4 Monatshefte fiir Chemie, 3, 257 (1882). 

5 Ber., 41, 3447 (1908). 


© Ber., 41, 3236 (1908); 42, 2997, 3881 (1909); Z. anorg. Chem., 67, 167 
(1910); 69, 158, 217 (1910); 75, 293 (1912); 82, 426 (1913). 


8s THE HYDROUS OXIDES 


constituent of a complex ion. It is interesting that, in the pres- 
ence of excess violet chromic acetate, iron and aluminum ace- 
tates cannot be detected either by heating to the boiling point 
or by adding caustic alkalies or ammonia. As will be pointed 
out later, hydrous chromic oxide peptized by hydroxyl ion 
adsorbs and so carries into colloidal solution a number of hydrous 
oxides not peptized by alkalies. This suggests that hydrous 
ferric oxide formed by hydrolysis of ferric acetate is kept from 
precipitating, owing to adsorption by colloidal hydrous chromic 
oxide. This suggestion does not seem to be in accord with the 
facts. In the first place, hydrous chromic oxide does not appear 
to be the primary product of the hydrolysis of chromic acetate; 
and in the second place, Reinitzer! showed that green chromic 
acetate formed by boiling and so hydrolyzing the violet salt 
does not prevent the precipitation of hydrous ferric oxide. This 
behavior of mixtures of ferric and violet chromic acetates is most 
likely due to the formation of one or more iron-chromic acetate 
complexes such as have been prepared by Weinland and 
Guzzmann.? 

Reinitzer? and Woudstra‘ claim to have made colloidal 
hydrous chromic oxide by dialysis of chromic acetate, but the 
extraordinary stability of the preparations in the presence of 
salts points to their being chromic acetate chiefly. This view is 
supported by the more recent attempt of Neidle and Barab® 
to dialyze a chromic acetate solution into which superheated 
steam was passed. Although such a procedure would favor the 
growth of any particles of colloid, all the chromium passed through 
the membrane. 

It is not possible to prepare a chromic oxide sol by dialysis of 

pure chromic chloride in the cold;® but the commercial salt yields 
a dilute sol. The difference in behavior is probably due to 
acceleration of hydrolysis of the commercial chloride by the 
presence of a little colloid as impurity.’ Since the temperature 

17. Chem. Soc., 42, 825 (1882). 

2 Ber., 42, 3881 (1909). 

3 Monatshefte fiir Chemie, 3, 249 (1883). 

4 Kolloid-Z., 5, 33 (1909). 

5 J. Am. Chem. Soc., 38, 1961 (1916). 


6 NEIDLE and BaraB: J. Am. Chem. Soc., 39, 71 (1917). 
7 GoopWIN and GRovER: Phys. Rev., 11, 193 (1900). 


HY DROUS CHROMIC OXIDE 89 


coefficient of hydrolysis of chromic chloride is considerable, ! 
very much higher yields are obtained by dialysis at 75 to 80°. 
The colloids are clear, deep green, and perfectly mobile when 
first prepared; but they gel on standing if the dialysis is carried 
too far. 

Since nitrate ion usually has a smaller precipitating action on 
positive sols than chloride ion, Biltz? attempted. to prepare a 
number of colloids by dialysis of nitrate solutions. This met 
with little success in the case of chromium nitrate, on account of 
the relatively small hydrolysis constant of the salt.* 

The Negative Sol.—If an excess of alkali hydroxide is added 
in the cold to a chromic salt solution, the precipitate first formed 
is peptized completely, giving a clear green colloidal solution. 
In this respect, hydrous chromic oxide differs from hydrous 
aluminum oxide which dissolves in alkali hydroxides, giving 
aluminate.‘ The sol formed in this way precipitates spontane- 
ously on standing,°® particularly if the ratio of oxide to hydroxyl] 
ion is too large. This is due to ageing of the hydrous oxide. 
For the same reason, a precipitated and washed oxide is not 
peptized by alkalies. The sol migrates to the anode under 
electrical stress,’ 1s precipitated by low concentration of salts 
having strongly adsorbed cations,* and the oxide particles can 
be removed by an ultrafilter.° 

Further evidence of the colloidal nature of an alkaline solution 
of hydrous chromic oxide is given by its action with other hydrous 
oxides. Thus Northcote and Church!® observed that complete 


1 Bserrum: Z. physik. Chem., 69, 343 (1907). 

2 Ber., 35, 4431 (1902). 

3 WoupstTRA: Kolloid-Z., 5, 33 (1909). 

4 HILDEBRAND: J. Am. Chem. Soc., 35, 864 (1913). 

5 Wiscuer and Hey: Z. anorg. Chem., 31, 352 (1902); Herz: Jbid., 28, 
344 (1901); 32, 357 (1902). 

6 Hantzscu: Z. anorg. Chem., 30, 338 (1902); cf., however, Herz: Z. 
anorg. Chem., 28, 344 (1901). 

7KREMANN: Z. anorg. Chem., 33, 87 (1903). 

8 Fiscuur: Z. anorg. Chem., 40, 39 (1904); cf., however, KREMANN: [bid., 
33, 87 (1903). 

9 Nace: J. Phys. Chem., 19, 331, 569 (1915); Bancrorr: Chem. News, 
113, 113 (1916); Trans. Am. Electrochem. Soc., 28, 351 (1915); CHatTrEeRJ1 
and Duar: Chem. News, 121, 253 (1920). . 

10 J, Chem. Soc., 6, 54 (1853). 


90 THE HYDROUS OXIDES 


solution takes place when chromic oxide is associated with 40 
per cent of ferric oxide, 12.5 per cent of manganous oxide, and 
20 per cent of either cobalt or nickel oxide; and that complete 
precipitation takes place when chromic oxide is associated with 
80 per cent of ferric oxide, 60 per cent of manganous oxide and 
with 50 per cent of either cobalt or nickel oxide. Similar 
observations were made by Prud’homme! and by Kreps.? This 
anomalous behavior is readily explained in view of the colloidal 
nature of alkaline solutions of chromic oxide. The colloidal 
oxide adsorbs to a limited degree and so carries into colloidal solu- 
tion the hydrous. oxides of iron, manganese, cobalt, nickel, 
copper, and magnesium, thus preventing their precipitation. 
The latter oxides likewise adsorb chromic oxide and so tend to 
take it out of colloidal solution in alkali. Accordingly, if they 
are present in sufficient amount, they will carry down and so 
decolorize practically completely the green colloidal solution 
of chromic oxide.* As previously noted, the behavior of ferric 
acetate in the presence of violet chromic acetate cannot be 
accounted for in this way. 

Wood and Black‘ treated precipitated chromic oxide with 
alkali of varying concentrations. After 2 months some chro- 
mate was found in solution, suggesting that chromic oxide dis- 
solves in alkali with the formation of chromite. Since one should 
expect this action to be more marked if the oxide were in the 
colloidal state, the author added a large excess of alkali to a 
chromic chloride solution and allowed the mixture to stand 2 
months. Most of the colloid precipitated in this time, leaving a 
greenish-yellow supernatant liquid, the yellow color being due 
to a small amount of chromate formed by oxidation of chromite 
in the air. While these observations show the oxide to possess 
a slightly acidic character,® they do not mean that the solution 
of hydrous chromic oxide in alkali is all chromite which subse- 
quently decomposes, as Wood and Black imply. Onthecontrary, 
it is extremely doubtful whether any chromite at all is formed 


1 Bull. soc. chim., (2) 17, 253 (1872). 

2 Thesis, Berlin (1893). 

3 NAGEL: J. Phys. Chem., 19, 231 (1915). 

4 J. Chem. Soc., 109, 164 (1916). 

® Cf. also MUuunr: Z. physik. Chem., 110, 363 (1924), 


HYDROUS CHROMIC OXIDE 91 


within a reasonable time in the presence of slightly more than 
enough alkali to cause complete solution.! 


PRECIPITATION AND ADSORPTION 


A well-dialyzed chromic oxide sol prepared by the Graham 
method is very sensitive to the presence of electrolytes, particu- 
larly if they contain multivalent cations.2 The precipitation 
values of a number of potassium salts for a sol containing 3.65 
grams Cr.O3 per liter are given in Table VI. It will be noted 
that iodate behaves like a multivalent ion in having a very high 
precipitating power. It is not known why this should be, since 
a dilute solution of iodic acid acts like a monobasic acid.* It is 


TaBLeE VI.—PRECIPITATION VALUES OF SALTS 


Precipitation Precipitation 
Potassium value, milli- Nature of Potassium value, milli- Nature of 
salt equivalents | precipitate salt equivalents | precipitate 
per liter per liter 
Ferricyanide... 0.485 Firm jelly || Bromate...... 19.0 Jelly 
Chromate...... 0.525 Firm jelly || Chloride...... 30.0 Jelly 
Dichromate.... 0.535 Firm jelly || Bromide...... 33.0 Jelly 
SU Tatec toes. 3 0.550 Firm jelly || Chlorate...... 33.8 Jelly 
Oxalate........ 0.570 Firm jelly || Iodide........ Ml EP Jelly 
hodate.. ase... % 0.635 Firm jelly 





particularly interesting that the conditions at the precipitation 
concentration are favorable to the formation of a jelly. This 
point will be considered in a later section. The jellies can be 
broken up by shaking, yielding gelatinous precipitates. 
Influence of Hydrogen Ion Concentration.—Everett E. Porter 
has recently made some precise precipitation and adsorption 
studies on a chromic oxide sol at varying hydrogen ion concentra- 
tions. As an illustration, some precipitation experiments with 
oxalate are given in detail in Table VII; and some adsorption 
data for the same ion are recorded in Table VIII and shown 


1 Cf. WaisEer: J. Phys. Chem., 28, 428 (1922). 

* Cf. Brerrum: Z. physik. Chem., 110, 678 (1924). 

§ Miouati and Mascetti: Gazz. chim. ital., 31, I, 93 (1901); Atti accad. 
Lincei, (5) 14, I, 217 (1905). 

4 Unpublished results. 


92 THE HYDROUS OXIDES 
TaBLe VII.—PREcIPITATION OF CHROMIC OxIDE SOL AT VARYING PH 
VALUES 


Cubic centimeters solution 
mixed with 5 cubic centi- 


meters sol, total volume | Precipitation value, poe 
20 cubic centimeters milliequivalents per 
liter 
N/50 HsC.Os\N/50 KyC.0s Belore yaa 
mixing mixing 
3.00 0.0 Seao 3.05 3.19 
R15 0.2 3.80 3 out 3.08 
2.80 0.4 3.20 
2.60 0.5 3.10 3.17 3.40 
2°20 0.7 2.90 
2.00 0.8 2.80 3.38 3.51 
1.60 0.9 2:50 
0.00 1.0 1.00 9.40 5.79 


TABLE VIII.—ApbDsorPTION OF OXALATE BY Hyprous CHROMIC OXIDE AT 
VARYING PH VALUES 


Cubic centimeters solution mixed with 








50 cubic centimeters sol, total volume | Adsorption pH values 
200 cubic centimeters values, milli- 
equivalents per 
gram Cr.O3 
N/50 HiC;04|N /50 K,C20s|N'/50 KOH ae 
mixing | mixing 
50 0 0 4.88 PY 2.89 
40 10 0 4.77 2.87 3.16 
30 20 0 4.47 3.16 3.78 
20 30 0 4.17 3.81 4.56 
10 40 0 3.48 4.45 | 7.54 
0 50 0 2.06 9.40 8.73 
0 50 5 ty 10.58 8.99 
0 50 10 0.65 10.67 9.20 
0 50 20 0.00 LL 9.48 
0 50 30 0.06 1x23 9.99 





HY DROUS CHROMIC OXIDE 93 


graphically in Fig. 5. The sol prepared by Graham’s method 
was dialyzed until the hydrogen ion concentration was but little 
greater than that of water. The precipitation values at vary- 
ing pH values for a preparation containing 2.5 grams Cr.03 per 
liter, were determined by mixing 5 cubic centimeters of sol with 
15 cubic centimeters of solutions containing salt and acid. The 
pH values “after mixing’”’ were made on the supernatant liquid 
after precipitation. A like solution diluted with water instead 


fey) 





Oo 


Adsorption in Milliequivalents per Gram Cr,03 


Fia. 5.—Influence of pH on adsorption of oxalate by hydrous chromic oxide. 


of with the colloid was used to get the pH values ‘‘ before mixing.” 
The two determinations are, of course, not strictly comparable. 
The adsorption values were obtained by mixing 50-cubic-centi- 
meter portions of sol with 150-cubic-centimeter portions of the 
several solutions and analyzing the supernatant liquid after 
precipitation. A rapid increase in precipitation value occurs 
between pH = 6 and pH = 3.5 in the solution after mixing. 
Above pH = 2.5, the adsorption of oxalate decreases quite 
rapidly with increasing pH value and becomes zero when the 
pH value after mixing is approximately 9.5. It is obvious, 
therefore, that both hydroxyl ion and oxalate ion are adsorbed 


94 THE HYDROUS OXIDES 


on the alkaline side of the isoelectric point, the carrying down of 
oxalate being completely nullified only in the presence of a 
relatively high concentration of hydroxide ion. 

Precipitating Action of Mixtures of Electrolytes.—Thirty 
years ago Linder and Picton! made the interesting observation 
that the precipitating action of mixtures of pairs of electrolytes 
for colloidal arsenious sulfide was not additive in case the elec- 
trolytes have widely varying precipitating power.? Since this 
so-called ionic antagonism was not observed with gold sol and 
with von Weimarn’s*® sulfur sol which are anhydrous, but was 
observed with Odén’s‘ sulfur sol which is hydrous, Freundlich 
and Scholz® conclude that the hydration of the colloid and of 
the precipitating ions is of primary importance in producing 
ionic antagonism and so in determining whether the precipitating 
action of mixtures shall be additive or above the additive values. 
This leads to the deduction that arsenious sulfide sol is a hydro- 
phile, although it is not usually so considered; and to the sugges- 
tion that the behavior of colloids with mixtures is a suitable 
means of determining to what extent the stability is influenced 
by hydration. These conclusions are not in accord with some 
observations® on colloidal chromic oxide, a few of which are 
recorded in Table [IX and shown graphically in Fig. 6. 


TABLE 1X.—PRECIPITATION OF COLLOIDAL CHROMIC OxIDE BY MIXTURES 
OF ELECTROLYTES ; 


(Precipitation values in milliequivalents per liter) 








KCl + K,SO, | KCL + ° K.C,0, KSO, + KeC2Ou 
00.0 0.675 0.00 0.700 0.000 0.700 
12.5 0.425 12.5 0.460 0.250 0.430 
25.0 0.300 25.0 0.325 0.338 0.350 
37.5 0.185 37.5 0.200 0.500 0.175 
50.0 0.065 50.0) 0.070 0.675 0.000 
56.8 0.000 56.8 0 000 


1 J. Chem. Soc., 67, 67 (1895). 

2 Cf. Wiser: J. Phys. Chem., 25, 665 (1921). 

3 Von WEIMARN and MatyscHew: Kolloid-Z., 8, 214 (1911). 
4“T)er kolloide Schwefel’”’ (1912). 

® Kolloidchem. Bethefte, 16, 267 (1922). 

6 Weiser: J, Phys. Chem., 28, 232 (1924); 25, 665 (1921). 


HYDROUS CHROMIC OXIDE 95 


Although the sol is very highly hydrous, mixtures of elec- 
trolytes having widely different precipitating power, such as 
potassium chloride and potassium sulfate, do not give values 
considerably above the additive value, such as Freundlich and 
Scholz would predict. On the contrary, the values for such 
mixtures are actually less than additive by a quite appreciable 
amount. This is not unexpected in view of the fact that adsorp- 





Milliequivalents per Liter of KCL 
Milliequivalents per Liter of K,S0, 





Ss) 
Oo] 
~J¢ 


0 
Q 0.) 0.2 0.3 AN eee they oe) 
Milliequivalents per Liter of K25Q, and KC 20 


Fig. 6.—Precipitation of colloidal hydrous chromic oxide with mixtures of 
electrolytes. 


tion is relatively greater at lower concentrations. The adsorp- 
tion of chloride ion is proportionately greater at concentrations 
below its precipitation value so that relatively less sulfate or 
oxalate is necessary to bring the combined adsorption above the 
critical value necessary for neutralization and coagulation. Such 
a result would follow, however, only in case there is little or no 
antagonistic action between chloride ion and sulfate or oxalate 
ion in the sense that the presence of each decreases the adsorption 
of the other at concentrations below the precipitation value. 


96 THE HYDROUS OXIDES 


This is evident from the experiments recorded in Table X. Below 
the precipitation concentration, a relatively large amount of 
chloride has no effect on the adsorption of oxalate; and above 
this concentration, the adsorption of oxalate is cut down but 
4 per cent by 50 times its concentration of chloride and less than 


TABLE X.—ADSORPTION BY Hyprous CHROMIC OXIDE OF OXALATE IN THE 
PRESENCE OF CHLORIDE 


Mixtures added to 30 cubic centimeters PR eo 
colloid containing 0.06 gram Cr.O; XElE.Le BOSOT PS 
Cubic centi- 


N/2 KCI N/100 K2C.0, HO pene Grams per mol 


N/100 CnOs 

4.50 Lose 24.15 1.35 1.505 

3.00 1.95 25.05 105 2.174 

1.50 2.85 25 .65 2.85 SAPP 

0.00 4.204 25.80 4.20 4.682 

0.00 8.50 21.50 Tae 8.182 

21.50 8.50 0.00 6.66 7.424 
0.00 12.00 18.00 9.42 10.500 

18.00 12.00 0.00 8.83 9.842 
0.00 15.00 15.00 11.10 12.373 

15.00 15.00 0.00 10.68 11.905 
0.00 20 .00 10.00 12.46 13.889 

10.00 20 .00 0.00 12.35 13 .667 





2 Precipitation value. 


1 per cent by 25 times its concentration of chloride. It would 
appear, therefore, that the high precipitation value of potassium 
chloride is due to relatively weak adsorption of chloride ion 
associated with but slight adsorption of the stabilizing potassium 
ion. Similar behavior was noted with negative colloidal stannic 
oxide using mixtures of lithium chloride and either barium 
chloride or magnesium chloride, 


—— 


HYDROUS CHROMIC OXIDE 97 


For the sake of comparison, some results! on the precipitation 
of arsenious sulfide sol by mixtures are given in Table XI and 
represented graphically in Fig. 7. 


TABLE XI.—PRECIPITATION OF COLLOIDAL ARSENIOUS SULFIDE BY 
MIxTuRES OF ELECTROLYTES 


(Precipitation values in milliequivalents per liter.) 


LiCl + BaCl, 














NaCl + BaCl, KCl =e BaCl, 
00.0 1.60 00.0 1.60 00.0 1.60 
12.5 2.02 12.5 1.93 12.0 1.88 
25.0 2.18 25.0 joe} 25.0 1.92 
43.7 2.13 43.7 1.82 43.7 1.62 
62.5 1.78 62.5 1.30 62.5 1.05 
81.2 1.23 95.0 0.00 83 .0 0.00 
tae 2 0.00 


HCl + _ BaCl, | KCl aa CeCl; KCl + NaCl 








00.0 1.60 00.0 0.388 00.0 95.0 
12.5 1.98 12.5 0.230 25.0 64.0 
25.0 1.93 25 .0 0.162 50.0 30.0 
37.5 1.65 37.5 0.132 83 .0 00.0 
50.0 1.00 50.0 0.105 
61.5 0.00 62.5 0.067 

83 .0 0.000 





It will be seen that the precipitation values of salt pairs for the 
same sol, may be additive, may be greater than the additive 
values, or may be less than the additive values. From deter- 
minations of adsorption during the precipitation of sols, there 
appear to be three factors which determine the precipitation 
concentration of salt pairs; (1) the stabilizing action of the ion 
having the same charge as the sol; (2) the effect of each precipi- 
tating ion on the adsorption of the other; and (8) the relatively 
greater adsorbability of ions at lower concentration. Since 
the addition of small amounts of potassium ferrocyanide to 
ferrocyanide sols or of hydrochloric acid to hydrous oxide sols 


1 Weiser: J. Phys. Chem., 30, 28 (1926). 


98 THE HYDROUS OXIDES 


increases their stability toward electrolytes, Dhar! and Sen? 
are inclined to attribute all the so-called antagonistic action of 
salt pairs to adsorption of stabilizing ions. Thus, the addition 
of a small amount of potassium chloride to arsenious sulfide sol 
is believed to stabilize it, thereby increasing the precipitation 
value of barium chloride. But a small amount of potassium 
chloride does not stabilize arsenious sulfide sol in the same sense 
that hydrochloric acid stabilizes chromic oxide sol; hence, the 





Milliequivalents per Liter of BaCl, and CeCls 
Milliequivalents per Liter of KCL 





20 40 60 BO aie 
Milliequivalents per Liter of HCL, KCL,NaCl and LiCl 


Fie. 7.—Precipitation of colloidal arsenious sulfide with mixtures of electro- 
lytes. 


precipitation concentrations for mixtures of potassium and 
sodium chloride are additive; and for mixtures of potassium and 
cerium chloride, they are below the additive value. It would 
thus appear that the determining factor with mixtures of barium 
chloride and the alkali chlorides is the antagonistic action of 
each precipitating ion on the adsorption of the other, while with 
potassium and cerium chlorides, it is the relatively greater 
adsorbability of the precipitating cations below the precipitation 
value of either taken separately. In line with this view, the 
adsorption of barium ion is cut down much more strongly than 


1 Guosu and Duar: J. Phys. Chem., 29, 659 (1925). 
2Z. anorg. Chem., 142, 345 (1925). 


HYDROUS CHROMIC OXIDE 99 


cerium ion by the concentrations of alkali chlorides concerned. 
The precipitation values of such mixtures as NaCl and KCl, 
having a common anion, are additive since the adsorption of 
the precipitating cations are similar, and so the adsorption of 
each is affected but slightly by the presence of the other within 
the limits of the precipitation concentration. 

The adsorbability of the alkalies deduced from their power to 
cut down the adsorption of barium ion is in the order: K > Na > 
Li.t From the curves in Fig. 7 it will be seen that the precipita- 
tion value of barium chloride is increased by like amounts of 
alkali chlorides in the order LiCl > NaCl > KCI; while in the 
presence of HCl, the precipitation value of barium salt first 
rises to a point just below that in the presence of a like amount of 
lithium chloride and then drops off rather sharply. The follow- 
ing explanation of these phenomena is suggested: For a given 
alkali chloride concentration, precipitation will take place when 
the combined adsorption of the two cations neutralizes the 
combined adsorption of chloride and hydrosulfide ions. The 
combined adsorption will be equivalent for different pairs of 
cations; but the relative amounts of each that make up this 
equivalent adsorption will vary, depending as it does on the 
relative adsorbability of the two cations. If one may disregard 
for the moment the slight variation in the amounts of chloride 
added with barium chloride as compared with the relatively large 
amount of this ion added with the alkali chloride, it follows that, 
for a given concentration of different alkali chlorides, the vary- 
- ing amounts of barium that must be added will depend on the 
effect of each cation on the adsorption of the other. Thus, the 
adsorption of barium is cut down by lithium ion less than by 
potassium ion, tending to make the precipitation concentration 
of barium chloride less in the presence of lithium chloride than of 
potassium chloride. Hand in hand with this is the decrease in- 
the adsorption of alkali by barium, which will tend to make the 
precipitation concentration of barium chloride higher in the 
presence of lithium. From this point of view, the latter factor 
appears to predominate with the alkali chlorides. With hydro- 
chloric acid, however, the cutting down of the adsorption of 
barium by hydrogen ion is the determining factor with lower 

1 Weiser: J. Phys. Chem., 29, 955 (1925); cf. p. 125. 


100 THE HYDROUS OXIDES 


concentrations of hydrochloric acid; while with higher concen- 
trations of acid, the second factor appears to predominate. 

Precipitation of Negative Sol—In Table XII are given the 
precipitation values of several electrolytes for a negative sol 
prepared by mixing 5 cubic centimeters of chromic chloride con- 
taining 40 grams Cr.O; per liter with 45 cubic centimeters of 
0.2N KOH. The precipitation value is that concentration of 
electrolyte which will just cause complete coagulation in 10 min- 
utes. It will be noted that the precipitating power of cations 
follows the usual order: barium > lithium > sodium > potassium; 
and the stabilizing action of the an‘ons is: sulfate > chloride > 
acetate. 


TABLE XIJ.—PRECIPITATION VALUES OF SALTS 


Precipitation value, 


Salt milliequivalents gil x, nf 

erliee precipitate 

Barium, chloride... 4.0 ee See Gelatinous 
Potassium ehlorides:. .. ace. oe 500.0 Gelatinous 
BOUL ChlOTIde. <4 5 fee a eee 210.0 Gelatinous 
Lithnim chioridet:s ter fee 51.0 Gelatinous 
sodium: sulfates 5 5a. eee een 315.0 Gelatinous 
Sodim-acetate.. 10) oo ee 220.0 Gelatinous 


Chromic Oxide Jellies—Mention has been made of Reinit- 
zer’s observation that a solution of chromic salt boiled with 
sodium acetate and rendered alkaline with caustic alkalies or 
ammonia sets to a jelly. Bunce and Finch! confirmed this 
observation and showed further that a jelly is formed by adding 
excess sodium hydroxide or potassium hydroxide to chrome alum 
and allowing the solution to stand. They were unable to obtain 
a jelly from chromic sulfate, nitrate, or chloride; but Nagel? 
succeeded in getting a jelly with sulfate by keeping down the 
concentration of alkali. From these observations it was logical 
to conclude that acetate or sulfate ions are necessary for the 
formation of a chromic oxide jelly. That such is not the case 

1Cf. J. Phys. Chem., 17, 769 (1918). 


2 J. Phys. Chem., 19, 331 (1914). 
3 BancroFT: ‘Applied Colloid Chemistry,’’ 244 (1921). 


——s1 = 


HYDROUS CHROMIC OXIDE 101 


is evident from the series of experiments recorded in Table XIII, 
using chromic chloride instead of sulfate or acetate. The experi- 
ments bear out the general conclusions regarding jelly formation 
previously considered in detail.!. The rapid addition of a slight 
excess of alkali to a chromic chloride solution produces a negative 
colloidal oxide that is instable and precipitates slowly, forming a 
jelly (Table XIII). If this precipitation is hastened by heating 
or by addition of a suitable amount of electrolyte, the precipitate 
forms so rapidly that it is gelatinous and not jelly-like (Table 
XII). Finally, if the hydrous oxide has been peptized by too 
great a concentration of alkali, the precipitate comes down very 
slowly and is almost granular in character, as observed by Nagel. 


TaBLE XIII.—Curomic Ox1DE JELLIES FROM NEGATIVE COLLOID 


Solutions (cubic centi- 
meters) mixed 











Alkali Observations aes 
Alkali | icy, | Total AA 
0.6 N volume 
ING OE eee 10.0 5.0 25 Peptization incomplete Gelatinous 
Na Ho. heres 11.5 5.0 De Peptization incomplete Firm green jelly 
Na OMe tes 11.75 5.0 25 Peptization almost complete | Firm green jelly 
NOE a pnantl 12.0 5.0 25 Peptization complete Firm green jelly 
[OIEL ANS es eee 10.0 5.0 25 Peptization incomplete Firm green jelly 
CO Has tees 10:75 on0 25 Peptization almost complete | Firm green jelly 
HCO epee aes 11.0 5.0 20 Peptization complete Firm green jelly 
Ba(OH) seemed. 20.0 baO Do No peptization Gelatinous 
ISAO sae vs ae 24.0 1.0 25 No peptization Gelatinous 
INS OH. fhe > 5. 1355 5.0 50 Peptization almost complete | Soft green jelly 
INS OH ae aaaes eve 5.0 50 Peptization complete Soft green jelly 
i OT ete ee aes 12.0 5.0 50 Peptization almost complete | Soft green jelly 
OMT a leg: ashes. | Pagers a0 50 Peptization complete Soft green jelly 


The experiments under consideration corroborate the observa- 
tion of Fischer and Herz that the peptizing power of potassium 
hydroxide is slightly greater than that of sodium hydroxide. 
On the other hand, they disprove the statement that hydrous 
chromic oxide is peptized by barium hydroxide and that the pep- 
tizing power of alkalies depends on the absolute amount present 
and not on the concentration. The hydroxides arranged in 
order of peptizing power are potassium hydroxide > sodium 
hydroxide > barium hydroxide. As would be expected, this is 


1 See p. 26, et seq. 


102 THE HYDROUS OXIDES 


the reverse of the order of precipitating power of the cations 
(Table XII). 

Knowing the conditions favorable to jelly formation by pre- 
cipitation of a negative colloidal hydrous chromic oxide, it is a 
simple matter to precipitate the positive sol as a jelly. All 
that is necessary is to add just enough electrolyte to cause com- 
plete coagulation in an hour or two. If too little electrolyte is 
used, precipitation is incomplete and the results are unsatisfac- 
tory; while if too great an excess is added, the precipitation is so 
rapid that a gelatinous precipitate is formed. From the results 
recorded in Table X, it is quite evident that jellies will form in 
the presence of any precipitating ion. Moreover, the hydrogen 
ion concentration within which jellies will form, can vary over a 
wide range; thus, they are obtained from strongly alkaline solu- 
tion and from a colloid stabilized by hydrogen ion. 

A typical jelly containing but 0.18 per cent chromic oxide will 
stand for days without undergoing noticeable syneresis. Shaking 
destroys the jelly structure, which does not re-form as in the case 
of more concentrated jellies.! 


1 Of, ScHALEK and Szecvary: Kolloid-Z., 32, 318; 33, 326 (1923). 


CHAPTER IV 


THE HYDROUS OXIDES OF ALUMINUM, GALLIUM, 
INDIUM, AND THALLIUM 


Hyprovus ALUMINUM OXIDE 


The Gelatinous Oxide.—The addition of ammonia to an 
aluminum salt solution throws down a very highly gelatinous 
precipitate of hydrous aluminum oxide. An x-radiogram of the 
precipitate formed in the cold with not too dilute solutions 
shows it to possess no crystalline character! even after prolonged 
drying at room temperature.? The precipitate exhibits a wide 
variation in properties depending on the conditions of formation 
and the age and history of the sample. Thus Tommasi’ found 
the newly formed oxide to be quite soluble in acids and alkalies, 
whereas the aged product was sparingly soluble. Recently 
Willstatter and Kraut* described a number of hydrous oxides 
differing in reactivity and adsorptive power, by precipitating 
aluminum sulfate with ammonia: With concentrated ammonia, 
and boiling for a long time, the precipitate was a pale yellow 
plastic mass A; without prolonged boiling, it was a very pale 
yellow plastic mass B; with dilute ammonia it was a pure white, 
very voluminous, and very finely divided substance C. An 
intermediate variety 6 prepared by the dialysis of aluminum 
chloride with frequent additions of small quantities of ammonia, 
was claimed to be related chemically to B but resembled A in 
adsorptive capacity; and a modified form of C precipitated at 
60° had an adsorptive capacity similar to B. Specimens of A 
were entirely different in properties, depending on whether they 


1Haser: Ber., 55, 1727 (1922); cf. Frickz and WEAVER: Z. anorg. 
Chem., 136, 320 (1924). 
2 Boum and Nicuassen: Z. anorg. Chem., 182, 1 (1924); Boum: Jbid., 
149, 203 (1925). 
3 Compt. rend., 91, 231 (1880); cf. PHituips: Phil. Mag., (3) 38, 357 (1848). 
4 Ber,, 66, 149, 1117 (1923); 57, 58, 1082 (1924), 
103 


104 THE HYDROUS OXIDES 


were still moist or subjected to a rapid preliminary drying in a 
high vacuum over P,O;. As a result of desiccation experiments, 
Willstatter concluded that the different gels contained a variety 
of different hydrates. This brings to mind earlier papers on 
hydrous aluminum oxide in which are described such hydrates 
as Al,O3; - H.O! corresponding to the crystalline mineral diaspore, 
Al,O; -2H,O? corresponding to amorphous bauxite, and Al.O3:- 
3H.O* corresponding to crystalline gibbsite; but the existence 
of hydrates in gelatinous alumina is rendered doubtful by the work 
of Carnelley and Walker® and of van Bemmelen.* The latter 
showed that at constant temperature the precipitated oxide takes 
up or gives off water until the vapor tension of the substance is 
the same as that of the surroundings; hence, change in tempera- 
ture causes a continuous change in the water content of the 
substance by varying its vapor tension. Moreover, the vapor 
pressure of the hydrous oxide is influenced by the conditions of 
precipitation and the subsequent treatment. Thus an oxide 
adsorbs water more strongly if thrown down from a dilute solu- 
tion of aluminum chloride than from a concentrated solution. 
The precipitate decreases in solubility in alkali and acids in pro- 
portion to the quantity of water lost by heating; after heating at 
various temperatures, the different oxides adsorb smaller quan- 
tities of water when placed in a saturated atmosphere, and they 
retain less in dry air in proportion to the water lost. By standing 
under water, the capacity to adsorb water and the solubility in 
acids and alkalies alters in proportion to the time of standing 
An ‘“‘amorphous” hygroscopic oxide formed by ageing the gelat- 
inous precipitate for 6 months under water and drying in air, 


1 MirscHERLicu: J. prakt. Chem., 83, 468 (1861); BecqurrEt: Jahresber., 
87 (1868); Ramsay: J. Chem. Soc., 32, 395 (1877). 

2 Lowen: Z. fiir Chemie, 3, 247 (1864); P&an de St. GinuEs: Ann. chim. 
phys., (3) 46, 57 (1856); Crum: Liebig’s Ann. Chem., 89, 156 (1853). 

3 ALLEN: Chem. News, 82, 75 (1900); Cossa: Z. fiir Chemie, 18, 4438 
(1873); Tommasi: Compt. rend., 91, 231 (1880). 

4Other hydrates have been described by ZuNINOo: Gazz. chim. ital., 30, 


194 (1900); Ramsay: J. Chem. Soc., 32, 395 (1877); ScHLUMBERGER: Bull. . 


soc. chim., (3) 18, 41 (1895). 

6 J. Chem. Soc., 58, 87 (1888). 

6 Rec. trav. chim., 7, 75 (1888); cf. Surper: Mem. Coll. Sci., Kyoto, 9a, 42 
(1924). 


aa ee | a a ay 


ALUMINUM, GALLIUM, INDIUM, AND THALLIUM 105 


corresponds to a trihydrate in composition. Van Bemmelen’s 
general conclusions were confirmed by observations on gelatinous 
alumina by Martin! and Kohlschiitter? and on ‘‘fibrous”’ alumina? 
_by von Zehman;* but Willstatter claims that vapor-pressure data 
do not show the absence of hydrates in preparations dried in 
vacuum over P.O; or treated with acetone, operations which are 
tacitly assumed to remove all adsorbed water. On the contrary, 
Willstatter’s predried preparations give certain temperature 
intervals of almost constant water content which are cited to 
prove the existence of hydrates. Thus the acetone-dried oxide 
precipitated from aluminum sulfate at low hydroxyl ion con- 
centration analyzes approximately for a trihydrate (van Bem- 
melen); and the precipitates obtained with excess ammonia in 
the hot give what are assumed to be polyaluminum hydroxides, 
such as 2Al1(OH)3 - H2O, as a result of intermolecular dehydration. 
Willstatter believes fresh gels to be hydrates; but his arguments 
are vague and unconvincing. Thus he says: 


It is not known whether the hydrates found after desiccation existed 
originally in the gelatinous suspension with the same amount of chem- 
ically combined water. Of course, there is no need of assuming the 
existence of single hydrates. The formulas calculated for hydrates of 
alumina in the cases described are complex; they have little significance, 
for they can usually be looked upon as mixtures of different hydrates. 
Whatever may be the water content and the degree of hydration of 
the gel suspended in water, it follows from the drying curve of prepa- 
ration C, (Al(OH); -nH:O which dries to Al(OH);), that desiccated 
preparations with values between (A103). - H20 and Al.O3 - H2O could 
not be Al(OH); in the original moist condition, but are probably mix- 
tures of compounds of the composition AI,(OH)3, - #H.0, 12.e., 
polymetahydroxides. 


Willstatter’s observations on a variety of oxides would seem to 
disprove the existence of hydrates with the possible exception of 
van Bemmelen’s trihydrate. Wide variations in the conditions 
of forming the oxides cause differences in their physical character 
and structure that determine not only the behavior toward 


1 Mon. sci., (5) 5, 225 (1915). 

2Z. anorg. Chem., 105, 1 (1919); Z. Elektrochem., 29, 246 (1923). 
3 WIsLICENUS: Z. angew. Chem., 17, 805 (1904). 

4 Kolloid-Z., 27, 233 (1920). 


106 THE HYDROUS OXIDES 


reagents and their adsorption capacity for dyes and enzymes, 
but also the amount of water they retain under given conditions. 
The nature and the location of the kinks or bends in the tempera- 
ture-dehydration curves depend on the previous history of the 
sample and so are different for each sample. As might be 
expected, the composition of a preparation treated in a certain 
definite way may sometimes be represented by a Dalton formula; 
but this does not prove the existence of a definite compound. 
Van Bemmelen! obtained breaks in the vapor-pressure curves 


Pressure 
WwW 
pape oes 
Si ey es ey 
Temperature 


(pe) 
Oo 
oO 





Minutes 
Fia. 8.— Dehydration of hydrous alumina prepared at 15°. 


for gels of lower water content; but these were not due to the 
presence of hydrates, for the location of the breaks varied with 
the history of the sample, and similar breaks were observed when 
alcohol or benzene was substituted for water. 

Guichard? followed the continuous dehydration of hydrous 
aluminas with increasing temperature by means of a specially 
designed hydrostatic compensation balance.* The form of the 
curves is well illustrated by Figs. 8 and 9 showing the results of 
experiments carried out on the gelatinous oxide precipitated (1) 


1 “T)ie Absorption,’’ 257 et seq. (1910). 


2 Bull. soc. chim., 37, 381 (1925). 
3 GUICHARD: Bull. soc. chim., 37, 251 (1925). 


ALUMINUM, GALLIUM, INDIUM, AND THALLIUM 107 


in the cold and (2) at the boiling point. With regular increase 
in temperature there appears a slight slowing down of the 
dehydration of the oxide formed in the cold, between 150 and 
200°, corresponding to a composition between 3H.O and 2H,0. 
The ‘‘pseudo flat” is interpreted to indicate the existence of 
Al,03:3H2O with adsorbed water and possibly Al,O3-2H2O with 
adsorbed water. Contrary to what one might have expected, the 


Pressure 
Temperature 





Minutes 


Fia. 9.—Dehydration curve of hydrous alumina prepared at 100°. 


aged crystalline oxide formed in the hot gives no indication of 
being Al.O3;-3H2O or any other hydrate. | 

The Crystalline Hydrate.—Although the oxide formed by 
precipitation of aluminum chloride with ammonium hydroxide 
contains no definite hydrates, crystals of artificial gibbsite have 
been prepared in a number of ways. Bornsdorff! obtained such 
a compound by saturating a sodium hydroxide solution with ' 
gelatinous alumina and allowing the solution to stand in a closed 


1Pogg. Ann., 27, 275 (1833); cf. VAN BEMMELEN: Rec. trav. chim., 7, 75 
(1888); Bayer: Chem. Ztg., 12, 1209 (1889); Dirre: Compt. rend., 116, 183 
(1893); AtLEN: Chem. News, 82, 75 (1900); Russ: Z. anorg. Chem., 41, 
216 (1904), 


108 THE HYDROUS OXIDES 


vessel.! A similar compound is formed by passing carbon dioxide 
through a boiling solution of alkali aluminate; by boiling alumi- 
num in water for many hours; by the action of hydrogen peroxide 
on aluminum;? by the action of water on aluminum amalgam;? 
by allowing potassium aluminate and aluminum chloride to 
mix slowly through a diaphragm ;* by calcining hydrated alumi- 
num nitrate;> and by electrolysis of an aqueous solution of 
alum.* According to Milligan? the composition of this com- 
pound remains constant up to 145°, when it starts losing water 
continuously with increasing temperature. All but 8 per cent 
of the water is driven off below 200°, and there is no evidence 
whatsoever of another hydrate. Alumina dried at as low a 
temperature as 275° takes up water by adsorption but does not 
combine to reform the hydrate. The higher the temperature of 
ignition the less the adsorption capacity of the oxide, doubtless 
on account of decreased porosity resulting from sintering. 
Alumina prepared from amalgamated aluminum is much denser 
than the ordinary precipitated hydrous oxide.° 

The gelatinous oxide freshly precipitated in the cold dissolves 
in acids and alkalies forming salts and is readily peptized by 
certain dilute acids and salts. The precipitate thrown down 
from the hot solution is less reactive and less easily peptized. 
The newly formed oxide ages fairly rapidly in the hot and more 
slowly in the cold even under water. Two modifications of 
alumina have, therefore, been recognized, the so-called ordinary 
or alpha and meta or beta modifications, representing the two 
extremes of reactivity. But there is no temperature of inversion 
from the soluble alpha to the insoluble beta form; on the con- 
trary, between these two extremes one may have all possible 
variation in solubility, reactivity, and adsorbability depending 
on the structure of the mass which in turn is determined by the 
conditions of precipitation and the subsequent method of treat- 


1 WoutER: Liebig’s Ann. Chem., 118, 249 (1859). 

2 WELTZIEN: Liebig’s Ann. Chem., 138, 130 (1866). 

3 Cossa: Z. fiir Chemie, 18, 448 (1873). 

4 BECQUEREL: Compt. rend., 67, 1081 (1868). 

5 ScHLésine: ‘Traite d’analyse,”’ Paris, 105 (1877). 

6 Duxtuo: Bull. soc. chim., (2) 5, 78 (1866). 

7 J. Phys. Chem., 26, 247 (1922); cf. Martin: Mon. sci., (5) 5, 225 (1915). 
8 Haun and Tureuer: Ber., 57, 671 (1924). 


ALUMINUM, GALLIUM, INDIUM, AND THALLIUM. 109 


ment.! These conclusions have been confirmed by Kohlschiitter? 
with different pseudo crystals of hydrous aluminas formed by 
the action of ammonia on crystals of aluminum salts. 

Unlike the fresh gelatinous oxide, the crystalline trihydrate 
is almost insoluble in cold acids and alkalies; it is very slowly 
soluble in hot concentrated HCl but it dissolves readily in con- 
centrated H.SO,. It is, therefore, similar in properties to the 
aged gelatinous oxide. By means of x-radiograms Bohm and 
Niclassen* observed the gradual transformation from an amor- 
phous to a crystalline oxide during ageing. Naturally, this raises 
the question whether the ageing process consists essentially 
in the gradual formation of crystalline trihydrate. X-radio- 
grams would appear to answer this in the negative, for aged 
oxides, obtained by precipitation in the hot or by precipitation 
in the cold and heating to 100° for an hour,‘ gave interference 
patterns corresponding to the indefinite mineral bauxite’ and 
not to definite crystalline gibbsite.6 However, from observations 
of the magnetic susceptibilities of a number of freshly precipi- 
tated and aged gels, as well as of crystalline trihydrate from 
potassium aluminate, Pascal’ concludes that newly formed gels 
consist solely of anhydrous Al,O3; with adsorbed water. On 
long standing, the gels go over to van Bemmelen’s unstable 
trihydrate, which appears to be quite distinct from the crystalline 
trihydrate of the same composition obtained from aluminate. ~ 
We know definitely that gelatinous alumina, aged in the presence 
of alkali, gives trihydrate crystals identical with gibbsite; but 
neither x-ray nor magnetic analyses furnish conclusive evidence 
as to whether van Bemmelen’s submicrocrystalline oxide, formed 
by ageing the ammonia-precipitated oxide in the cold, is really a 
trihydrate, and if so, whether it is identical with artificial gibbsite 
or an allotropic modification of the latter. For the present I 
am inclined to attribute the difference between the crystalline 
oxides aged by long standing in cold water and aged in dilute 

1 WaisEeR: J. Phys. Chem., 24, 505 (1920). 

2Z. anorg. Chem., 105, 1 (1919). 

$Z. anorg. Chem., 182, 1 (1924). 

4Miuuican: J. Phys. Chem., 26, 254 (1922). 

6 Fricke and Weaver: Z. anorg. Chem., 186, 321 (1924). 


6 RINNE: Z. anorg. Chem., 136, 322 (1922), 
7 Compt. rend., 178, 481 (1924), 


110 THE HYDROUS OXIDES 


alkali to a difference in specific surface rather than in composi- 
tion or chemical structure. Five years ago, before x-ray analysis 
established the crystalline character of aged aluminum oxides, 
Fricke! observed a marked difference in the physical character 
and solubility of trihydrates obtained from aluminate under 
varying conditions. 

The adsorption capacity of a gelatinous oxide aged for a short 
time is much greater than that of crystalline trihydrate from 
aluminate; and the heating curves of the two are quite distinct.? 
On heating the crystalline trihydrate, there appears a diminution 
corresponding to an endothermal change below 360°; while 
gelatinous alumina gives a curve with a decided hump at 850°, 
corresponding to an exothermal change. Mellor and Holdcroft* 
suggest the term calorescence for the exothermic phenomenon. 
This calorescence or glow phenomenon, like that observed by 
calcining hydrous chromic oxide, is a manifestation of the 
energy lost by a sudden diminution of surface at some tempera- 
ture. The relatively large trihydrate crystals which precipitate 
from alkali aluminate do not caloresce when heated, since they 
possess a much smaller surface for a given mass than the aged 
gelatinous oxide. The diminution in surface of the heated 
oxide is accompanied by a decrease in hygroscopicity, specific 
gravity, reactivity, and adsorbability. X-radiograms* show that 
' the ignited oxide is not a different allotropic modification. 

Anhydrous Alumina. Corundum Gems.—While there is 
little evidence of the existence of a and 6 hydrous oxides of 
aluminum, the anhydrous oxide has been prepared in two dis- 
tinct forms:> a aluminum oxide, the usual trigonal, crystalline 
form represented by corundum; and 8 aluminum oxide formed 
in hexagonal crystals or appearing in groups of overlapping 
triangular plates when a aluminum oxide is melted and allowed 
to cool slowly. The presence of a small amount of MgO 
(0.5 per cent) materially assists the formation of 6 aluminum 
oxide while small amounts of either calcium oxide or silicon diox- 


1Z, Elektrochem., 26, 143 (1920). 

2 Le CHATELIER: Compt. rend., 104, 1517 (1887). 

3 Trans. Ceram. Soc., 10, 169 (1912); 138, 83 (1914). 

4 HepvALL: Z. anorg. Chem., 120, 327 (1922). 

5 RANKIN and Merwin: J. Am. Chem. Soc., 38, 568 (1916). 


ALUMINUM, GALLIUM, INDIUM, AND THALLIUM 111 


ide favor the formation of the a variety. _Since the 8 form does 
not revert to the a form even when held at temperatures above or 
below the melting point, it is suggested that 6 aluminum oxide 
may be monotropic with respect to the a form; but this con- 
clusion does not follow. 

The existence of these two modifications of alumina is of inter- 
est in connection with the color of corundum gems. ‘The color 
of pure Al,O; is white or water clear. Natural corundum occurs 
as blue, green, violet, yellow, and white sapphires and as ruby 
which varies in color from pale rose to carmine red or bluish red. 
The yellow, purple, and green sapphires are sometimes called 
oriental topaz, amethyst, and emerald, respectively. The 
pleochroism is marked in some gems. Thus the ruby may be 
deep red in the direction of the vertical axis and lighter color or 
colorless at right angles to this direction. Similarly, the sapphire 
may be deep blue in the direction of the vertical axis and greenish 
blue to bluish white when viewed at right angles. The various 
tints are due to the presence of colored oxides. By fusing alu- 
minum and chromium oxides Fremy and Verneuil! synthesized ruby 
and also obtained crystals which, in parts, had the color of blue 
sapphire.” The difference in color was attributed to a difference 
in the state of oxidation of the chromic oxide. If such is the case, 
it would appear that the ruby may owe its color to CrO3 and the 
sapphire to Cr.O3. In line with this, a red color can be obtained 
only in an oxidizing atmosphere; moreover, by heating a ruby 
in a reducing atmosphere it may become green or even color- 
less,? owing to the low tinctorial power of the green oxide. There 
are, however, two difficulties with this hypothesis. In the first 
place, CrO; is instable at the temperature of molten alumina, 
and so we must make the unproved assumption that the oxide 
is stabilized by alumina; and in the second place, we do not know 
whether CrO3; when highly dispersed will give a red color to 
alumina. An alternative hypothesis is that the different colors 
of gems with chromic oxide as pigment are due to variation in 
the size of the particles of Cr203. While this would account for 


1 Compt. rend., 111, 667 (1890). 

2 Cf. Deviiute and Caron: Compt. rend., 66, 765 (1858). 

3 BortTer: “ Edelsteinkinde,”’ Leipsig, 88 (1893); KENNGoTT: Neues 
Jahrb. Mineral, Geol., 313 (1867); Rinne: [bid., I, 170 (1900); II, 47 (1906). 


112 THE HYDROUS OXIDES 


variations in color from light blue to dark green,! it seems unlikely 
that this explanation can be extended to include the red color. 
The Norton Company found that. artificial gems made with 
a Al,O3; and chromic oxide are red, while those made with 8 
oxide are green. Bancroft suggests, therefore, that the different 
colors are due to different allotropic modifications of Cr2Q3. 
Since a alumina is only partly converted into 8 alumina by melt- 
ing and slowly cooling the oxide, Bancroft’s explanation might 
account for red and blue patches in the same crystals, both natural 
and artificial. 

Morozenwicz? claimed to get rose, yellow, greenish-yellow, 
red, and pale-blue corundum with iron oxide and so suggested 
that the coloring agent in certain gems is due to iron instead of 
chromium oxides. Verneuil’s* most recent work on the synthesis 
of sapphires leads him to attribute the coloration of natural 
sapphires to iron and titanium oxides. However this may be, 
there is no denying that artificial cee may owe their color 
to chromium oxide. 


ALUMINUM OXIDE SOLS 


Since it is possible to prepare an indefinite number of hydrous 
aluminas differing in the size and structure of the particles and 
the amount of water they contain, it is possible to obtain col- 
loidal solutions of alumina having widely varying properties 
depending on the method of formation. Two general methods 
of preparation are employed: hydrolysis of aluminum salts, and 
peptization of the hydrous oxide by acids and salts. 

Hydrolysis of Aluminum Salts.—Gay Lussac‘* boiled a con- 
centrated solution of aluminum acetate and obtained a precipi- 
tate of hydrous alumina which redissolved when the temperature 
was lowered. Crum heated a more dilute and more basic solu- 
tion than Gay Lussace, first in a closed vessel and subsequently 
in an open one, to drive off the excess acetic acid. In this way a 


1 Weiser: J. Phys. Chem., 26, 417 (1922). 

2 Tschermak’s mineralog. petrog. Mitt., (2) 18, 456 (1899), 
3 Compt. rend., 150, 185 (1910). 

4 Ann. chim. phys., (1) 74, 193 (1810). 

* Liebig’s Ann. Chem., 89, 168 (1854), 


ALUMINUM, GALLIUM, INDIUM, AND THALLIUM 113 


stable but opalescent colloidal solution was formed, containing 
alumina and acetic acid in the ratio of 5.5:1. The conditions 
of formation, namely prolonged digestion at high temperature 
with subsequent boiling in a medium having a slight solvent 
action, were conducive to the formation of relatively large dense 
non-reactive primary particles. Accordingly, the oxide thrown 
down from the sol by electrolytes was an aged coagulum made up 
of crystalline particles that were not very soluble in acids or 
alkalies and had no mordanting action. 

Graham! prepared a sol having properties similar to Crum’s 
by heating an acetate solution for several days and then dialyzing 
in the cold. The time required for making Crum’s sol may be 
materially shortened by peptizing freshly precipitated hydrous 
alumina with the smallest possible amount of acetic acid, diluting, 
and boiling to remove the excess acid.2,_ Minachi and Okazaka* 
diluted a saturated solution of aluminum acetate in dilute acetic 
acid, added hydrogen dioxide, and dialyzed at 50 to 80°. 
Attempts to prepare colloidal alumina by dialysis of the chloride 
and nitrate’ in the cold have not proved successful, owing to 
the relatively low degree of hydrolysis of even !4999 M solutions.°® 
Since the temperature coefficient of the hydrolysis is quite high,*® 
Neidle’ was able to get a 9.5 per cent conversion of a 0.05 M 
solution of AlCl; by dialyzing for 37 hours at 75 to 80°. 

Peptization of Hydrous Alumina.—Graham! peptized freshly 
prepared and thoroughly washed hydrous alumina in a solution 
of aluminum chloride and then dialyzed out the excess of the 
peptizing agent in the cold. By this method a positively charged 
sol results that is very sensitive to the action of electrolytes. 
The precipitate formed on coagulation is highly gelatinous, is 
readily soluble in acids and alkalies, and isa mordant. The sol, 
therefore, bears the same relation to Crum’s colloidal alumina 


1Phil. Mag., (4) 28, 290 (1862); see also ScHLUMBERGER: Bull. soc. 
chim., (3) 18, 62 (1895). . 

2 Weiser: J. Phys. Chem., 24, 525 (1920). 

3 Japanese Patent 41726 (1922). 

4 Bintz: Ber., 35, 4432 (1902). 

5 Ley: Z. physik. Chem., 30, 219 (1899). 

6 BserRuM: Z. physik. Chem., 59, 343 (1907). 

7 J. Am. Chem. Soc., 39, 71 (1917). 

8 Liebig’s Ann. Chem., 121, 41 (1862). 


114 THE HYDROUS OXIDES 


that Graham’s colloidal ferric oxide bears to the Péan de St. 
Gilles sol. Analogous to ferric oxide sols, the difference in prop- 
erties of the two colloidal aluminas is closely associated with the 
size and physical character of the hydrous particles. Peptization 
of highly gelatinous alumina in the cold favors the formation of 
small highly hydrous primary particles that are more reactive 
and have a higher adsorption capacity than the more granular 
and denser particles formed during prolonged boiling in a medium 
possessing a slight solvent action. 

The peptization of an alumina gel by AICI; does not take place 
very readily; but Hantzsch and Desch! got around this difficulty 
by adding ammonia to an aluminum chloride solution until the 
precipitate first formed failed to dissolve, and then dialyzing the 
sol. By evaporating the transparent purified sol on the water 
bath, a glassy mass was obtained which was readily repeptized 
by water; but the new colloid was quite opalescent owing to the 
formation of larger crystalline? particles during the process of 
evaporation. The sol prepared by hot dialysis was also slightly 
opalescent, possessing properties intermediate between Graham’s 
and Crum’s sols. 

Highly purified sols cannot be prepared by adding ammonia to 
aluminum sulfate and dialyzing, because of the precipitating 
action of sulfate ion. However, one may add NaeCO; and 
Al,(SO,)3 in the approximate ratio of 3:5 without any precipi- 
tation taking place; when the ratio is 7.5:5, half the alumina is 
thrown down; and when it is 12:5, all the alumina precipitates.’ 

Schneider‘ first peptized gelatinous alumina with a dilute 
solution of HCl. The excess acid was removed by evaporating 
to dryness and repeptizing with water. The sol gave no test for 
chloride ion with AgNO; in the cold, but AgCl precipitated out 
on heating; with silver oxide, both AgCl and the sol were thrown 
down. The failure to get a test for chloride ion in the cold was 
doubtless due to inhibition of the growth of AgCl particles by 
the protecting power of hydrous alumina. Heating caused the 
AgCl to show up, owing to partial agglomeration of the particles. 


1 Tiebig’s Ann. Chem., 328, 30 (1902). 

2 Boum and NicLassENn: Z. anorg. Chem., 182, 1 (1924). 
3 Miuits and Barr: J. Chem. Soc., 41, 341 (1882). 

4 Tiebig’s Ann. Chem., 257, 359 (1890). 


ALUMINUM, GALLIUM, INDIUM, AND THALLIUM 115 


The addition of silver oxide introduced the strongly adsorbed 
hydroxyl ion which neutralized the charge on the particles 
precipitating the colloidal ‘oxide together with AgCl. Miller! 
boiled freshly prepared hydrous alumina with N/20 HCl and 
found the amount of acid required for complete peptization to 
be one-seventy-second of that necessary to form AICl;; Pauli? 
used one-ninth of the theoretical amount; and Kohlschiitter? 
showed that the quantity of acid required was determined by 
the history of the sample. The dissolution of hydrous alumina 
in concentrated HCl is always preceded by sol formation; but 
H.2SO, does not form a sol. 

Hydrous alumina is peptized by ferric chloride or nitrate but 
not by ferric sulfate. The peptizing action of the chloride and 
nitrate is due to strong adsorption of ferric ions and of hydrogen 
ions resulting from hydrolysis of the salts. Such sols contain 
both hydrous alumina and hydrous ferric oxide. With ferric 
sulfate, the peptizing action of the cations is neutralized by strong 
adsorption of sulfate ion and no sol is formed. The order of 
peptizing power of different acids and salts on an aged gel thrown 
down from a boiling solution is: HNO; > HCl > FeCl > 
AICl, > HC2H30..* 

If we assume, as Lottermoser does, that a peptizer must contain 
one of the ions of the disperse phase, then the first step in the 
peptization of alumina by an acid or salt would be interaction 
with the formation of some aluminum ion. This would seem to 
be an unnecessary step in view of the stronger peptizing action 
of hydrogen ion than of aluminum ion. On account of the 
relatively small ionization of acetic acid, its peptizing power is 
less than that of HCl or HNO;. Bentley and Rose® have 
reported many anomalies in the behavior of the sol formed by 
peptizing alumina with acetic acid; but for the most part, these 
are the result either of experimental error or of misinterpretation 
of data.® 

1Z. anorg. Chem., 57, 311 (1908); cf. SCHLUMBERGER: Bull. soc. chim., (3) 
13, 60 (1895). 

2 Kolloid-Z., 29, 281 (1921). 

3 Z. Elektrochem., 29, 253 (1923). 

4 Weiser: J. Phys. .Chem., 24, 521 (1920). 

5 J. Am. Chem. Soc., 35, 1490 (1913); Ros: Kolloid-Z., 16, 1 (1914). 

6 Weiser: J. Phys. Chem., 24, 522, 527 (1920). 


116 THE HYDROUS OXIDES 


On account of the marked tendency of aluminum salts to 
hydrolyze, one is not surprised to encounter a very large number 
of basic aluminum chlorides, sulfates, and acetates. While 
there may be some definite salts of this type, it is certain that by 
far the most of them are mixtures of indefinite composition. ' 
Pauli and his collaborators? champion the view that the various 
alumina sols are highly complex basic salts of variable composi- 
tion. While one cannot deny the possible existence in a sol of 
such compounds as Pauli describes, there seems no reason for - 
postulating their existence until someone shows that such definite 
compounds are formed and defines their limits of stability. 

Action of Alkalies and Ammonia.—<As has been noted in the 
preceding chapter, hydrous chromium oxide is peptized by dilute 
alkalies forming a negative sol owing to preferential adsorption 
of hydroxyl ion, and little or no chromite results within a reason- 
able time. Certain investigators are of the opinion that hydrous 
aluminum oxide is peptized in the same way and question the 
existence of definite aluminates in the alkaline solution. In 
support of this view, Mahin® observed the precipitation of a 
greater amount of hydrous alumina on adding NH,NO; to an 
alkaline solution of the hydrous oxide than is represented by the 
assumption that an aluminate is involved in the process, thus: 
NaAlO, +. NH,NO; — NH,AIO, aa NaNOs3; NH,AIO;, + 2H.O ee 
NH,OH + AI(OH)3. It was assumed, therefore, that NH:NO; 
merely coagulates a negative alumina sol. Moreover, in elec- 
trolysis the ratio between the oxygen evolved and the alumina 
precipitated should be always 1:2 if the solution were pure 
aluminate; but a slightly greater amount of alumina was obtained 
in some cases. As Blum‘ points out, Mahin’s observations are 
not conclusive because of the inability to control the spontaneous 
decomposition which is going on continuously in alkali solutions 
of alumina. Chatterji and Dhar’ observed no appreciable change 


1Orpway: Am. J. Sci., (2) 26, 196 (1858); Tommast: Bull. soc. chim., 
(2) 37, 443 (1882); ScotuMBERGER: Jbid., (3) 13, 60 (1895); LimcuTI and 
Suipa: Dinglers polytech. J., 251, 177 (1883). 

2 ApoLrF, Paul, and JANDRASCHITSCH: Kolloid-Z., 29, 281 (1921); 
Pau: I[bid., 28, 49 (1921). 

3 J. Am. Chem. Soc., 35, 30 (1913). 

4 J. Am. Chem. Soc., 85, 1499 (1913); 36, 2383 (1914). 

5’ Chem, News, 121, 253 (1920). 


ALUMINUM, GALLIUM, INDIUM, AND THALLIUM 117 


in the conductivity of solutions of alkali to which alumina was 
added; and so concluded that a sol was formed. Their observa- 
tions merely indicate the failure of their experimental method to 
detect any change in conductivity in the highly alkaline solu- 








Alkali, CC. 


Fia. 10.—Variation in pH on titration of aluminium chloride solutions with 
alkali. 


tion, rather than the absence of a change. The bulk of the 
evidence seems to indicate that alumina is acted on chemically 
- by alkali hydroxides. Thus Hildebrand and Blum followed the 
change in hydrogen ion concentration on adding NaOH or KOH 


1 Prescott: J. Am. Chem. Soc., 2, 27 (1880); Leyte: Chem. News, 51, 
109 (1885); Nores and WuitTney: Z. physik. Chem., 15, 695 (1894); Herz: 
Z. anorg. Chem., 25, 155 (1900); Hanrzscu: Jbid., 30, 289 (1902); RupEn- 
BAUER: Ibid., 30, 331 (1902); Suave: Jbid., 77, 457 (1912); Hi~pEBRAND: 
J. Am. Chem. Soc., 35, 864 (1913); Buum: /bid., 35, 1499 (1913); 36, 2383 
(1914); Snape and Poxack: Trans. Faraday Soc., 10, 150 (1914). 


118 THE HYDROUS OXIDES 


to a solution of aluminum salt until all the hydrous alumina is 
precipitated and subsequently dissolved. The results are repre- 
sented in Fig. 10, together with the neutralization curve for 
NaOH and HCl which is included for reference. With AICI; and 
NaOH, X represents the beginning of precipitation, Y the com- 
pletion of precipitation, and Z the completion of solution. From 
X to Y, 2AlCl; + 6NaOH + xH.0 = 6NaCl + Al,O3:2H.O, and 
from Y to Z, AleO3;:xH,O + 2NaOH = 2NaAlO2 + (@ + 1) 
H.O. Since yz is almost exactly l4zy, the formation of NaAlO» 
is rendered quite certain. 

The potassium salt KAIO.-15H.O has been isolated! in a 
crystalline form. The existence of alkali metaaluminates only 
has been established,? although other aluminates are believed 
to form under certain conditions.’ 

Even though alkalies act on hydrous alumina giving aluminate, 
the solutions always contain more or less colloidal alumina. 
Indeed, Kohlschiitter found that sol formation always precedes 
the dissolution of alumina from aluminate, and the same is 
probably true of aged hydrous alumina; but it is apparently 
not the case with gelatinous oxides. In any event, the aluminate 
undergoes progressive hydrolysis with the formation first of 
colloidal alumina, as Pascal‘ observed; and finally, of the erystal- 
line trihydrate which precipitates. According to Johnston,° this 
progressive hydrolysis with precipitation of alumina probably 
accounts for the strong alkalinity of solutions of alkali aluminates. 

In analytical chemistry it is well known that the precipitation 
of aluminum as hydrous oxide is not quantitative in the presence 
of an excess of ammonia® on account of the solvent action of the 

1 ALLEN and Roacers: Am. Chem. J., 24, 304 (1900). 

2 Woop: J. Chem. Soc., 98, 417 (1908); Carrara and VESPIGNANT: Gazz. 
chim. ital., 30 II, 35 (1900); Heyrovsxy: Chem. News, 125, 198 (1922). 

3 GROBET: J. chim. phys., 19, 331 (1922); Hmrz: Z. anorg. Chem., 25, 
155 (1900); Z. Elektrochem., 17, 403 (1911); GoupRtIaan: Rec. trav. chim., - 
41, 82 (1922). 

4 Compt. rend., 178, 481 (1924). 

> Private communication to BLum: J. Am. Chem. Soc., 35, 1503 (1913). 

6 FRESENIUS: “Quant. Chem. Analysis,” 2, 807 (1916); WepenuorsT: 
“‘Beitrage zur Quant. Bestimmung und Trennung des Aluminiums,”’ 
Gottingen (1921); Bum: Z. analyt. Chem., 27, 19 (1888); von WEIMARN: 


Kolloid-Z., 4, 38 (1909); JANDER and WEBER: Z. anorg. Chem., 131, 266 
(1923). 


ALUMINUM, GALLIUM, INDIUM, AND THALLIUM 119 


solution. Blum observed a small but appreciable solvent action 
when the solution is just alkaline to phenolphthalein at a hydro- 
gen ion concentration of 10-®. Whether this is due wholly or in 
part to peptization of the oxide is open to some question. When 
potassium aluminate is precipitated with the calculated amount 
of ammonium chloride and an excess of ammonia added rapidly, 
all the alumina redissolves. Renz! mixed the calculated amount 
of ammonium sulfate with barium aluminate to which an excess 
of ammonia was added, and after filtering off the barium sulfate, 
obtained a clear solution containing 2.0 grams Al,O3 per liter. 
On evaporating to dryness, there was found a white gummy mass 
of hydrous oxide insoluble in ammonia. The solvent action of 
ammonia is most pronounced at the moment of formation, and 
from analogy with the alkali aluminates it is probable that 
aluminate is formed.? Blum was unable to detect the presence 
of this salt by determing the change in hydrogen ion concentra- 
tion during the precipitation of hydrous alumina, because of the 
low alkalinity of the aqueous solution. If the salt exists, the 
maximum quantity that can be held in solution will be deter- 
mined by the alkalinity of the resulting solution and its ability 
to repress the hydrolysis of the salt. According to Archibald 
and Habasian, the solubility of alumina in ammonia rises to a 
maximum of approximately 0.45 gram per liter at a concentra- 
tion of about 0.5 N, and then decreases owing to a change in 
the physical character of the hydrous oxide. The solubility of 
Renz’s preparation in excess ammonia was more than four times 
this maximum. Ammonium nitrate decreases the solubility in 
ammonia while potassium nitrate apparently increases it. 
Lottermoser and Friedrich*® prepared a very readily peptized 
hydrous oxide by adding AICI; in small increments to a solution 
of N/10 NH,OH cooled to 0° and stirred by air saturated with 
ammonia. After thorough purification by dialysis, the oxide 
was peptized by AICI; slowly in the cold and rapidly at 60 to 
70°. Traces of ammonia peptized the gel forming a negative 
sol that was not very stable on heating. In the light of this 
work, Renz’s experiments should be repeated to determine the 
1 Ber., 36, 2751 (1903). 


2 Of. ARCHIBALD and Hagpasian: Trans. Roy. Soc. Canada, 10, 69 (1916). 
3 Ber., 57, 808 (1924). 


120 THE HYDROUS OXIDES 


nature of his solutions. Jander and Weber! found no evidence of 
sol formation on shaking precipitated alumina with ammonia 
solutions. For a given concentration of ammonia, the solubility 
was the same in the presence of monovalent and univalent anions; 
organic solvents have no precipitating effect;? and the solution 
passes readily through an ultrafilter. 

Alumina is not precipitated from an alum solution by ammonia 
in the presence of a tartrate owing to the formation of a complex 
aluminum tartrate.* A sol results by precipitation in the pres- 
ence of glucose. In some preliminary experiments on grinding 
alumina in a colloid mill with glucose, Utzino* claimed to get a 
sol, the maximum stability of which does not occur with the 
finest state of subdivision. These observations should be 
repeated. 

Coagulation of Sol.—The precipitating power of electrolytes 
for colloidal aluminum oxide sols has been studied repeatedly.°® 
While the absolute precipitation values of electrolytes vary with 
the concentration and purity of the sol and with the experimental 
method, the order is always approximately the same. On 
account of the transparency of the gelatinous oxide, some diffi- 
culty is experienced in determining the critical precipitation 
concentration of electrolytes. Kawamura took advantage of 
the change in viscosity which the sol undergoes on coagulation, 
and this method was adopted by Ishazaka and Gann. The latter 
in collaboration with Freundlich, followed the slow coagulation of 
colloidal alumina by the addition of electrolytes containing uni- 
valent precipitating ions. The process which was found to be 
autocatalytically accelerated, takes place in accordance with 


aoe ite : : 
the equation y = k(1 + bir) (1 — x) where z is the increase in 
viscosity after time ¢ expressed as a fraction of the total increase; 


1Z. anorg. Chem., 181, 266 (1923). 

2 YANEK: Ann. ecole mines Oural, 1, 45 (1919); Chem. Abstracts, 15, 1239 
(1921). 

3 Hakomori: J. Chem. Soc. Japan, 48, 629 (1922). 

4 Kolloid-Z., 32, 149 (1923). 

6 Kawamura: J. Coll. Sci., Imp. Univ. Tokyo, 28, Art. 8 (1908); IsHazaKa: 
Z. physik. Chem., 83, 97 (1913); Gann: Kolloidchem. Bethefte, 8, 125 (1916); 
Weiser and Mippueton: J. Phys. Chem., 24, 639 (1920); IwanrrzKasa: 
Kolloidchem. Bethefte, 18, 24 (1923). 


Cy a i 


ALUMINUM, GALLIUM, INDIUM, AND THALLIUM 121 


and k and 6; are constants. For concentrated sols the coagula- 


tion process is more nearly represented by the equation - = 


ki(1 — x)’. The coefficient k; increases rapidly with the con- 
centration of the electrolyte during slow coagulation, while for 
very rapid coagulation the velocity is independent of the nature 
of the electrolyte. In rapid coagulation, Smoluchowski! assumes 
that all the collisions of particles are inelastic because of the great 
attractive forces existing between particles; and in slow coagula- 
tion, only a portion of the collisions result in immediate union, 
because the mutual attraction is not always great enough to 
overcome the repulsive effect of more highly charged particles. 
Freundlich turns this around and assumes a constant force of 
attraction for a given concentration of electrolyte below that 
necessary for rapid coagulation; but because of repulsion between 
charged particles, only those collisions are inelastic in which the 
particles collide with sufficient force. Obviously, the greater 
the charge on the particles, the greater must be the velocity of 
collision in order to overcome the repulsive effect and so to 
bring about coalescence and agglomeration. The rapid increase 
in the velocity of slow coagulation is due to the proportionately 
larger number of inelastic collisions that result when the charge on 
the particles is reduced by adsorption of precipitating ions. 

Aluminum Oxide Jellies.—A sol formed by peptizing sufficient 
hydrous alumina to form a viscous liquid sets to a jelly on stand- — 
ing.? If this jelly is broken up by shaking, a gelatinous precipi- 
tate settles out which is not repeptized by the acid and so cannot 
be reconverted into a jelly. Schalek and Szegvary* prepared a 
so by Crum’s method which set to a jelly on the addition of a 
sulitable amount of electrolyte just below the precipitation value. 
This jelly was broken up on shaking, but instead of giving a 
gelatinous precipitate, a sol was re-formed that would again set 
to a jelly on standing. The reversible sol-gel transformation 
has been observed only with relatively concentrated sols of the 
hydrous oxide. A jelly may be formed by coagulating a dilute 
sol prepared by peptizing hydrous alumina with acetic acid; 


1Z. physik. Chem., 92, 129 (1917); Kolloid-Z., 21, 98 (1917). 
2 ScHLUMBERGER: Bull. soc. chim., (3) 13, 56 (1895). 
3 Kolloid-Z., 33, 326 (1923). 


122 THE HYDROUS OXIDES 


but shaking converts the jelly into a gelatinous precipitate that 
is not repeptized. 


ADSORPTION BY HYDROUS ALUMINUM OXIDE 


If an electrolyte is added to a sol stabilized by preferential 
adsorption of cations, precipitation will take place when the 
anions of the electrolyte are adsorbed sufficiently to reduce the 
charge on the particles below a critical value. Whitney and 
Ober! first showed that the amount of various ions carried 
down during the precipitation of arsenious sulfide sol are not far 
from equivalent. This conclusion was upheld by Freundlich? 
as a result of similar observations on adsorption during the 
precipitation of other sols. The results with alumina given in 
Table XIV are frequently offered as proof of equivalent adsorption 


TaBLE XIV 
Adsorption at the precipitation 
Precipitation values 
Ions value, millimols 
per liter Th yailltaeere In milli- 
equivalents 
Salicylate... J... + 8.0 0.30 0.30 
Pictates 4.0 0.18 0.18 
Oxalatese. 2. ee 0.36 0.18 0.36 
Ferricyanide...... 0.10 0.09 0:27 
Ferrocyanide...... 0.08 0.073 0.29 


during the precipitation of sols with precipitating ions of varying 
valence although the variation from equivalence is quite appreci- 
able. The investigations of Freundlich on aluminum oxide sol 
have been extended by Middleton’ with the results given in 
Table XV. The adsorption for different ions is not even approxi- 
mately equivalent, and the variation cannot be attributed 
to experimental errors, as Freundlich assumes. While the latter 
is doubtless right in concluding that neutralization of the charge 
is accomplished by adsorption of equivalent amounts, the 
1 J. Am. Chem. Soc., 23, 1842 (1901). 


2 “ Kapillarchemie,” 579 et seq. (1922). 
8 WEISER and MippLeTon: J. Phys. Chem., 24, 630 (1920). 


ALUMINUM, GALLIUM, INDIUM, AND THALLIUM 123 











TABLE XV 
Adsorption, Precipitation value 
Anion milliequivalents per | of salt, milliequiva- 
liter lents per liter 

(OC Gch Oe en 1.280 0.375 
Merrieyanide: si, v./,.....-.. oiLeQi4 0.400 
Se eee a ae 0.997 0.538 
Ci ene e: Aes) Mischa ia 1, 142 0.700 
SENCUIOALO te ee Sa. . 0.870 1.300 
POA eey ee ee ke. es 0.657 1.625 
LO OTe See en? a a 0.269 ier are: 


actual amount carried down is determined (a) by adsorption of 
the electrically charged particles during neutralization and (b) 
by adsorption of salt by electrically neutral particles during the 
agglomeration process. The amounts of (a) will be approxi- 
mately equivalent, but the amounts of (b) will vary with the 
nature and concentration of the electrolyte. Owing to salt 
adsorption by neutralized particles, Freundlich’s conclusion 
that equivalent amounts are adsorbed at the precipitation con- 
centration cannot be generally true, since this would mean either 
that the neutralized particles do not act as an adsorbent or 
adsorb all ions to the same extent. Moreover, the variability of 
the precipitation concentration will necessarily result in varia- 
tion in the degree of saturation of the adsorbent by the adsorbed 
phase. One should expect the adsorption value to approach 
equivalence more nearly, the less the adsorption capacity of the 
precipitated particles. This probably accounts for the values 
being more nearly equivalent with an arsenious sufide sol than 
with a hydrous oxide sol having many times the adsorption 
capacity. 

If the variation from equivalence arises from adsorption after 
neutralization, the adsorption values might appear a priori to 
give directly the order of adsorption of the ions. This is not 
necessarily true, however, because there are variable factors 
other than the adsorbability of the precipitating ions that deter- 
mine the amount of adsorption after neutralization; for example, 
the nature and degree of ionization and the degree of hydrolysis 
of the salt; the hydrogen ion concentration; the effect of different 


124 THE HYDROUS OXIDES 


salts on the physical character of the precipitate; ete. From the 
observations recorded in Table XV, the order of adsorbability 
expressed in equivalents would appear to be as follows: ferro- 
cyanide > ferricyanide > oxalate > sulfate > chromate > 
dithionate > dichromate. Considering the precipitation value 
of the several potassium salts, we find the order of precipitating 
power beginning at the greatest to be: ferrocyanide > ferri- 
cyanide > sulfate > oxalate > chromate > dithionate > 
dichromate. The order of adsorption determined directly is 
the same as the order deduced from precipitation data with the 
exception of oxalate and sulfate, which are reversed. The cause 
of this exception is not known; but in this connection, attention 
may be called to some unpublished work of Everett E. Porter 
which disclosed that the order of precipitating power of oxalate 
and sulfate for chromic oxide sol is determined by the hydrogen 
ion concentration of the precipitating solution. 

If the adsorption value is expressed in equivalents, as seems 
logical, since neutralization is determined by the number of 
adsorbed charges, the results given in Table XV are in accord 
with the usual interpretation of Schulze’s law that the ion of 
highest valence is most readily adsorbed. At the same time, the 
qualitative nature of the rule is indicated by the different adsorp- 
tion value for ions of the same valence. Schilow! found a wide 
variation in the adsorption of cations of the same valence by 
ignited alumina; but the ions of highest valence were most 
strongly adsorbed. 

It is unfortunate that a comparison of the relationship between 
the precipitation values of electrolytes and the adsorption of 
precipitating ions cannot be made directly with salts containing 
univalent precipitating ions since the precipitation values and 
adsorption values for multivalent ions are likely to be so close 
together that it is hazardous to draw conclusions, particularly 
when the differences may be of the same order of magnitude as 
the errors inherent in the experimental method. The direct 
determination of adsorption of univalent ions that precipitate 
only in high concentration, is impracticable since the change in 
concentration resulting from adsorption is too low to measure 
accurately. It is possible, however, to determine the relative 

1Z, physik. Chem., 100, 425 (1922). 


a ay 


ALUMINUM, GALLIUM, INDIUM, AND THALLIUM 125 


adsorbability of univalent precipitating ions during the precipi- 
tation of sols by an indirect method consisting essentially in 
determining the extent to which the presence of different uni- 
valent ions cuts down the adsorption of an easily estimated 
multivalent ion.! This is illustrated by the results recorded in 
Table XVI. The extent to which the adsorption of barium ion 


TaBLE XVI 





: : Barium adsorbed 
Cubic centimeters electrolyte added to Precipitation 


100 cubic centimeters sol, total volume Millienaivalents value, milliequiva- 
200 cubic centimeters. Grams lents per liter 
per gram 

SUN ASTOS VEMTILES ELS 5. ric i rr 0.0109 0.058 BaCle 274. 

30 N/50 BaCle + 30 N/2 LiCl........... 0.0037 0.019 LiCl 88.7 

30 NV /50 BaCle + 30 N/2 NaCl.......... 0.0025 0.014 NaCl (ono 

a0 750 BaCle -— 30 .N/2 KCl........... 0.0018 0.009 KCl 63.7 

30/50 BaCle-F 30):NV/2 HCl........... 0.0013 0.007 HCl SPAS 


by arsenious sulfide is cut down by the presence of the same 
amount of different alkali chlorides is in the order Li < Na < 
K < H. Since, under otherwise constant conditions, one should 
expect the adsorption of a given cation to be cut down by the 
presence of a second, in proportion to the adsorbability of the 
latter, it follows that the order of adsorbability of univalent 
ions is H>K>WNa>Ii. This is exactly the same as the 
order deduced from the precipitation values of the salts, assum- 
ing that the salt containing the most readily adsorbed cation 
precipitates in lowest concentration. 

Weak adsorption of the precipitating ions of electrolytes 
requiring a high concentration to effect neutralization, is indi- 
cated by the ease of reversibility of precipitation. Thus hydrous 
alumina precipitated from a sol with a relatively high concentra- 
tion of KCl, NaCl, or NaC.H3O: is readily carried back into 
colloidal solution by washing, whereas the precipitation is more 
nearly irreversible if K:SO, is the precipitating electrolyte. 
Similarly, a precipitated alumina thrown down from an alum 
solution with alkali can be washed until most of the sulfate 
is removed before peptization begins,? whereas the precipitate 
from chloride solution is very easily dispersed by washing. ‘The 


1 Weiser: J. Phys. Chem., 29, 963 (1925). 
2 BRADFIELD: J. Am. Chem. Soc., 44, 969 (1922). 


126 THE HYDROUS OXIDES 


difference in degree of reversibility of precipitation is determined 
by the relatively weak adsorption of univalent chloride as com- — 
pared with bivalent sulfate. Rakuzin' reports that hydrous 
alumina adsorbs gum arabic reversibly; but the adsorption 
from sodium and potassium silicate is partly reversible. 

The adsorption of chromate by hydrous alumina is sufficiently 
strong to impart a yellow color to the precipitate formed in the 
presence of alkali chromate or precipitated and subsequently 
shaken with alkali chromate solutions. Charriou? found little 
alkali metal in the precipitate and so attributed the color to the 
formation of aluminum chromate on the surface of the alumina. 
There is no justification for this conclusion and it is probably 
erroneous. If well-washed alumina is shaken with alkali chro- 
mate, the solution becomes alkaline owing to stronger adsorption 
of acid than of base. The yellow color is due to chromic acid 
rather than aluminum chromate. Ishazaka* found that potas- 
sium dichromate was converted to chromate in the presence of 
powdered alumina. The explanation of this phenomenon is as 
follows: The equilibrium in solution between dichromate and 
chromate ion may be represented by the equation Cr20,’’ + 
H,0 @ 2H’ + 2CrO,’’.. Alumina shows such a strong preferen- 
tial adsorption for hydrogen ion that the presence of the oxide in 
a finely divided condition shifts the equilibrium to the right with 
the formation of chromate ion at the expense of dichromate. 
Colloidal alumina stabilized by preferential adsorption of 
hydrogen ion has a comparatively slight effect on the equilibrium. * 

Adsorbed chromate is displaced but slightly by washing with 
5 per cent solutions of the more weakly adsorbed chloride, 
bromide, iodide, nitrate, or acetate; while chromate is displaced 
by ions the adsorption of which is of the same order of magnitude, 
such as carbonate, sulfate, sulfide, oxalate, tartrate, phosphate, or 
arsenate. Similarly, sulfate is not displaced by weakly adsorbed 
univalent ions but is displaced by bivalent ions. Charriou® 
generalized that an adsorbed ion is displaced by one of the same 


1 J. Russ. Phys.-Chem. Soc., 58, 357 (1921). 

2 Compt. rend., 176, 679, 1890 (1923). 

3Z. physik. Chem., 88, 97 (1918). 

4 Weiser and Mippieton: J. Phys. Chem., 24, 648 (1920), 
5 Compt. rend., 176, 1890 (1923), 


ALUMINUM, GALLIUM, INDIUM, AND THALLIUM 127 


kind having the same or a higher valence; but is not displaced 
by one of lower valence. With two ions of the same valence the 
less concentrated is displaced the most. These generalizations 
may be approximately true in certain cases, but they are neces- 
sarily not quantitative, since they are based on the erroneous 
impression that ions of like valences are all adsorbed to the 
same extent and that trivalent ions are always more strongly 
adsorbed than bivalent and bivalent ions always more strongly 
adsorbed than univalent. Generalizations based on Schulze’s 
law are of value only in so far as the limitations of Schulze’s 
law are fully recognized. ! : 

From the practical point of view, in the Pieniative estimation 
of alumina one may avoid the contamination by such ions as 
chromate by carrying out the precipitation with NH,HCOs; 
instead of NaOH, or one may remove the adsorbed ion by wash- 
ing the precipitate with NH,HCOs. 

Miller? investigated the simultaneous adsorption of sulfate 
and oxalate ions during the precipitation of alumina. The 
adsorbabilities of the two ions are not far apart, and it is claimed 
that the sum of the adsorption expressed in mols per mol of 
hydrous alumina may be considered constant, although the 
observed values really show variations of more than 20 per cent 
both above and below the mean value. The distribution ratios 

ion in solution 
ion in precipitate 
values for the same ion were considered to be of the same order 
of magnitude, although here the variations from a constant value 
were more than 100 per cent from the mean. As a result of 
these observations, the taking up of anions by the hydrous oxide 
was considered to be a solid-solution phenomenon. ‘This con- 
clusion seems hardly justified by the evidence. Of course, one 
cannot expect too much from data obtained on such a complex 
system where such factors as the hydrogen ion concentration 
and the physical character of the precipitate are not subject to 
control. But one cannot be certain of the effect of eliminating 
all variable factors other than the relative amounts of oxalate 
and sulfate. In case the adsorbabilities of the two ions are 


1 Weiser: J. Phys. Chem., 29, 963 (1925). 
2U. S, Pub, Health Repts., 39, 1502 (1924), 





at equilibrium were also calculated, and the 


128 THE HYDROUS OXIDES 


very similar, one should expect the total adsorption to be approx- 
imately equivalent, irrespective of the relative amounts of 
each; but if the adsorbabilities of the ions are widely different, 
there is likely to be an antagonistic action between the two 
which will cause the total adsorption to vary with the relative 
amounts of each in the solution. But even should the adsorp- 
tion be equivalent and the displacement follow the law of dis- 
tribution between solutions, it does not follow that the taking up 
of ions is a true solid-solution phenomenon rather than a surface 
phenomenon. If in certain cases there should be a reciprocal. 
displacement of adsorbed ions, there is probably no real objec- 
tion to calling the system a homogeneous single-phase solid 
solution as Miller does, provided one recognizes that this designa- 
tion is probably not strictly accurate. 

If a small amount of ferric salt is added to the test tube con- 
taining the precipitate thrown down from an alumina sol by the 
required amount of ferrocyanide, no Prussian blue is formed 
until after an appreciable interval of time.! This is not due to 
the slow rate of reaction between ferrocyanide and ferric ions as 
a result of the colloidal nature of ferric salt solutions;? but is due 
to the very strong adsorption of ferrocyanide ion which removes 
it from the field of action. If another strongly adsorbed precip- 
itating ion is added to the sol either before or after precipitation, 
the ferrocyanide is displaced and the time necessary for the 
appearance of Prussian blue is diminished appreciably. In 
the same way, the transformation of Congo blue to Congo red 
by dilute alkali is slowed down in the presence of hydrous alumina 
on account of the strong adsorption of Congo blue by the hydrous 
oxide.’ 

The selective adsorption of alumina seems to offer a great 
many possibilities to the biochemist,* although the observations 
to date are somewhat fragmentary. Rakuzin® reports that 
casein is adsorbed by alumina without splitting the molecule, 


1 RerrstoTrer: Kolloid-Z., 21, 197 (1917); Freunpuicw and Resrr- 
sTOTTER: Ibid., 23, 23 (1918). 

2 VORLANDER: Kolloid-Z., 22, 103 (1918). 

’ Bayuiss: Proc. Roy. Soc. London, 84, 81 (1912). 

4 KuLer-and Erikson: Z. physiol. Chem., 128, 1, 9 (1923). 

> Ber., 56, 1385 (19238); Z. Immunitdts., 34, 155 (1922), 


ALUMINUM, GALLIUM, INDIUM, AND THALLIUM 129 


whereas most proteins are broken up. Thus egg albumin is 
separated into two components differing in optical rotatory 
power; chondrin is separated from chondroitin sulfuric acid 
which remains in solution, while the colloidal chrondrin residue 
is adsorbed reversibly. Koch’s tuberculin and Deny’s tuber- 
culin can be distinguished by their difference in adsorbability. 
Alumina is also recommended for the: purification of pepsin 
and of diphtheria antitoxin; therapeutically, it is suggested for 
use in intestinal infections. 

It cannot be emphasized too strongly that comparative data on 
adsorption by hydrous alumina or any other substance cannot 
be obtained unless particular attention is paid to the physical 
character of the adsorbent. To make the most rapid progress 
it would seem to be essential for biochemists to get together on 
some well-defined arbitrary methods of procedure for making a 
series of preparations that could serve as standards. Thesystems 
with which the biochemists deal are so complicated at best that 
there seems no justification for carrying out adsorption experi- 
ments with adsorbents that are not standardized in some way. 

The important réle which hydrous aluminum oxide plays in 
the soil and in such important technical processes as water puri- 
fication and dyeing will be considered in later chapters. 


Hyprovus GALLIUM OXIDE 


Hydrous gallium oxide requires a very slight hydroxyl ion 
concentration for its precipitation and is thrown down in a 
highly gelatinous form not only by both strong and weak alkalies 
but by salts of weak acids,” such as carbonate, bicarbonate, 
sulfide, sulfite,* etc. ‘Tartaric acid prevents the precipitation, 
presumably because complex tartrates are formed.* Unlike 
hydrous alumina, the gel is fairly soluble in an excess of strong 
ammonia, doubtless owing to the formation of a complex gal- 
lium-ammonium ion; like alumina, it is very soluble in alkalies 
apparently forming gallates.> From the alkali solution, the oxide 


1EuLER and Nitsson: Z. physiol. Chem., 131, 107 (1923); 134, 22 (1924). 
2 Lecog DE BoisBAUDRAN: Chem. News, 35, 148, 157, 167 (1877). 

3 Dennis and BripGeMAN: J. Am. Chem. Soc., 40, 1531 (1918). 

4 Lmcog DE BoIsBAUDRAN: Compt. rend., 98, 293, 329, 815 (1881). 

5 Fricke and BLeNcKE: Z. anorg. Chem., 1438, 183 (1925). 


130 THE HYDROUS OXIDES 


precipitates very slowly out of contact with air, but it is readily 
thrown down by carbon dioxide as a flocculent mass entirely 
different from the granular crystals of AleO3:3H2O which precip- 
itate from aluminate solution.! 

Owing to the highly gelatinous nature of the precipitate, one 
should expect sol formation to result from thorough washing of 
the gel. Moreover, it:is not unlikely that a small amount of 
gallic chloride or hydrochloric acid would peptize the gel, forming 
a positive sol; or a slight excess of alkali, a negative sol; but there 
is no record of such experiments having been performed. There 
is some evidence that in the presence of excess alkali a part, at 
least, of the hydrous oxide is in the sol form. Thus, if COse is 
conducted into an alkali solution newly formed in the cold, one 
obtains a very voluminous gel, quite different in appearance and 
properties from the flocculent precipitate thrown down from 
an old alkali solution. Moreover, the solubility of the hydrous 
oxide in KOH solution is appreciably less than in NaOH solution,' 
as would be expected from the higher precipitating power of K’ 
ion than of Na’ ion for negative sols. 

Hydrous gallium oxide, like hydrous alumina, ages fairly 
rapidly even at ordinary temperatures, as evidenced by a pro- 
gressive loss of adsorbed water and a decrease in the solubility 
in alkalies.! As already intimated, the ageing progresses more 
rapidly in the presence of alkali. 

No definite hydrate of Ga2O3 has been established, and the 
available evidence indicates the non-existence of such com- 
pounds. The hydrous oxide precipitated from ammonia contains 
less water than corresponds to the hydrate Gae2O3 - 3H2O when 
dried in a vacuum desiccator over H2SQO, or heated on the water 
bath. The water content of an oxide obtained from an old 
alkali solution and dried over H2SOQ, is greater than required for 
a trihydrate; but the more gelatinous precipitate from a newly 
formed alkali solution falls considerably below that for a tri- 
hydrate, even when dried in the air at ordinary temperatures. ! 
Apparently, the hydrous precipitate is GazO; with adsorbed water 
in amount depending on the conditions of formation and the 
method of drying. 


1FRIcKE: Z. Elektrochem., 30, 393 (1924). 


ALUMINUM, GALLIUM, INDIUM, AND THALLIUM 181 


Hyprovus INDIUM OXIDE 


Hydrous indium oxide in a highly gelatinous form is precipi- 
tated by adding alkali, ammonia, hydroxylamine,! or dimethyl- 
amine? to a solution of an indium salt. The oxide loses water 
continuously on heating, and there is no indication of the existence 
of a hydrate.* The last traces of the adsorbed water are not 
removed until a temperature of 650° is reached; but the oxide 
undergoes no appreciable decomposition below 850°.‘ Like 
hydrous gallia and alumina, the precipitate ages slowly at ordi- 
nary temperature but rapidly at the boiling point, particularly 
in the presence of alkali. The newly formed oxide dissolves in 
the cold in excess alkali but soon precipitates out in a much less 
reactive form.> This precipitation is almost quantitative in 
the hot. Unfortunately we do not know whether in the cold 
the oxide is dissolved by excess alkali forming an indate or 
whether it is peptized forming a negative sol, although the latter 
seems more likely. Nor do we know whether the precipitate 
which comes down on standing is an aged hydrous oxide as in 
the case of chromic oxide or a definite hydrate as in the case of 
alumina. These problems should be investigated. 

As ordinarily prepared, hydrous indium oxide is but slightly 
soluble in ammonia. Renz’ claimed at one time to have obtained 
an ammonia-soluble form of the gel; but later he was not sure 
about it. It is, of course, altogether possible that the hydrous 
oxide may be thrown down under special conditions in a more 
soluble or more readily peptizable form than that ordinarily 
obtained; and if so, there should be little difficulty in determining 
whether a sol is formed, as has been suspected.® 

Whatever may be the nature of the alkali solution of hydrous 
indium oxide, the gel is readily converted into a sol by thorough 
washing with distilled water. Even the denser gel thrown 

1 Dennis and Guer: Ber., 37, 961 (1904). 

2 Renz: Ber., 34, 2763 (1901); 36, 1847, 2751, 4394 (1904); 87, 2111 

1904). 

3 eee and WALKER: J. Chem. Soc., 58, 88 (1888). 

4THiEL and Kornscu: Z. anorg. Chem., 66, 288 (1910). 

5 Mryer: Liebig’s Ann. Chem., 150, 137 (1869). 

6 RicHaRps and Borer: J. Am. Chem. Soc., 41, 133 (1919). 

7 Ber., 36, 1848, 2754 (1903). 

8TuieL: Z. anorg. Chem., 40, 322 (1904); cf., however, Ture, and 
Korxscu: [bid., 66, 300 (1910). 


132 THE HYDROUS OXIDES 


down at 100° is peptized in this way. The colloidal solutions 
obtained by Thiel precipitated out in a few weeks’ time; but there 
is no doubt that very stable sols could be formed by supercentrif- 
ugal washing. To prevent sol formation during quantitative 
washing, it is only necessary to follow the time-honored practice 
of adding a little ammonium salt to the wash water. 

A stable colloid is easily obtained by passing air through a 
cold solution of indium monoiodide.t The reaction 2InI + 
xH2O + Oe = In20; - cH2O + 2HI goes slowly, practically all 
of the oxide remaining colloidally dissolved. If desired, there 
seems no reason why this sol should not be purified by dialysis. 

By carrying out the oxidation of indium monoiodide in the 
hot, 99 per cent of the indium is converted into hydrous oxide 
most of which precipitates out. Obviously, the oxide ages quite 
rapidly, or it would not precipitate from dilute acid solution in 
which the newly formed gel is very soluble. Indeed, the oxide 
thrown down in this way is almost insoluble in the cold in dilute 
acids and dissolves but slowly in concentrated ones, a behavior 
analogous to the ageing of the better-known hydrous oxides of 
aluminum and chromium. 

Anhydrous indium oxide likewise furnishes a good example of 
the influence of the physical character of an oxide on its chemical 
properties. Not only is an oxide heated to 850° acted on much 
more readily than one ignited at 1200°;? but a newly formed 
oxide decomposes into In304 and Oe» between 1200 and 1500° 
much more rapidly than a dense preparation aged by long igni- 
tion’ at a low red heat. 


Hyprovus THALLIC OXIDE 


The most hydrous form of thallic oxide is obtained by adding 
ammonia or alkali in slight excess to a thallic salt solution in the 
cold. The very insoluble* voluminous precipitate is reddish 
brown in color like hydrous ferric oxide; it adsorbs alkali strongly, 
and in consequence, ammonia is always used as the precipitant 
in the estimation of thallium as trioxide.® If the solution after 

1 Turret and Koruscu: Z. anorg. Chem., 66, 300, 304 (1910). 

2 Renz: Ber., 36, 1848 (1903); Ture, and Koruscu: Z. anorg. Chem., 66, 
296 (1910). 

3 THIEL: Z. anorg. Chem., 40, 322 (1904). 

4 ApnaG and Spencer: Z. anorg. Chem., 44, 379 (1905). 

6 Mpyer: Z. anorg. Chem., 24, 364 (1900). 


ALUMINUM, GALLIUM, INDIUM, AND THALLIUM 1338 


precipitation is heated to boiling, the brown mass of hydrous 
oxide loses practically all its water becoming a dark granular 
powder; in this respect it behaves like blue hydrous cupric oxide. 
The oxide precipitated in the cold and dried in the air has a 
composition approaching Tl,O3;-H2O.! It has, therefore, been 
tacitly assumed that the red-brown slimy precipitate is a mono- 
hydrate. While this may be true, it is probably purely accidental 
that the composition at room temperature can be formulated 
Tl,03-H2O, particularly since it loses water continuously above 
room temperature, becoming almost anhydrous at 100°. 

By treating an alkaline solution of a thallous salt with hydrogen 
peroxide at room temperature,* one obtains a dark-brown floc- 
culent precipitate of hydrous oxide which changes slowly to 
small lustrous crystals of almost anhydrous Tl.O3. If the reac- 
tion is carried out at 80 to 100°, the oxide is a black sandy pow- 
der. ‘The density of the black oxide is 5.6 per cent higher than 
the brown; and the latter dissolves much more readily in acids 
and is more readily reduced to the thallous state by boiling water. 
It is probable that these difference in properties are due entirely 
to variations in the physical character of the mass, as determined 
by the conditions of formation, and not to allotropy as assumed 
by Rabe.* Indeed, by heating the brown oxide to 500° the pri- 
mary particles sinter together and assume permanently the prop- 
erties of black oxide. 

A crystalline hydrate of thallic oxide, ThO3:3H2O or TI1(OH)s;,° 
stable to a temperature of 340°, is said to be formed by prolonged 
fusion of Tl,O3; with KOH and subsequently treating the yellow 
mass with water. ‘These observations should be repeated, as 
the formula was derived from thallium analyses the accuracy of 
which is not known. 

Thallous oxide forms a definite crystalline hydrate, Tl,O -H2O 
or TIOH, soluble in water and possessing basic properties of 
the same order of magnitude as the caustic alkalies. 

1 CARNELLEY and WALKER: J. Chem. Soc., 58, 88 (1888). 

2 BIRNBAUM: Liebig’s Ann. Chem., 138, 133 (1866); WmrruEr: J. prakt. 
Chem., (1) 91, 385 (1864); cf., however, Lamy: Compt. rend., 54, 1255 (1862) ; 
58, 442 (1863). 

3 RaBe: Z. anorg. Chem., 48, 427 (1906); 50, 158 (1906). 


4Z. anorg. Chem., 55, 130 (1907). 
6 CARNEGIE; Chem, News, 60, 113 (1889). 


CHAPTER V 


THE HYDROUS OXIDES OF COPPER, COBALT, NICKEL, 
SILVER, AND GOLD | 


Although the compounds of cobalt and nickel are usually 
studied in connection with those of iron, it seems advisable to 
consider their hydrous oxides along with copper. ‘Thus, the 
most common oxide of iron is ferric oxide, whereas the most 
common oxides of nickel and cobalt are the ‘‘ous”’ oxides which 
are more nearly related to cupric oxide than to ferrous oxide. 
Moreover, the relationship between the blue and rose oxides of 
‘cobalt is similar in certain respects to that between the blue and 
black oxides of copper. 


Hyprovus Cupric OXIDE 


The gelatinous mass obtained by the addition of dilute alkali 
to a cuprous salt is usually considered to have the composition 
Cu(OH.)z or CuO: H20. This is because the precipitate, washed 
rapidly until free from the mother liquor and dried over H2SQOu, 
has the composition corresponding to a monohydrate. Van 
Bemmelen! found the freshly precipitated blue substance to be 
highly hydrous, containing more than 20 mols of water to 1 of 
cupric oxide even after pressing between porous earthenware for 
2 hours. When the precipitate is exposed at ordinary tempera- 
tures to an artificially dried atmosphere, it loses water con- 
tinuously until the vapor pressure is equal to that of the 
atmosphere. From +2H2O to +1H20O, the water is held more 
firmly than above +2H,0O, and at a pressure of zero, the oxide 
approaches the properties and composition of a crystalline 
hydrate, CuO-H.O. The ease with which water is eliminated 
decreases with the age of the sample; but even the freshly pre- 
pared oxide does not lose all its water at 100°. Neither a dihy- 
drate nor a trihydrate is formed,” and van Bemmelen considers 

1Z. anorg. Chem., 5, 466 (1894). 

2 Cf, Sprina: Z. anorg. Chem., 2, 195 (1892). 

134 


COPPER, COBALT, NICKEL, SILVER, AND GOLD 135 


the evidence insufficient to establish the existence of a definite 
amorphous oxide of the composition CuO : H20; hence, the gelat- 
-inous body must be looked upon as hydrous ale oxide rather 
than hydrous hydrated cupric oxide. 

Besides the familiar blue gelatinous oxide, a crystalline com- 
pound can be obtained in a number of ways. Becquerel! pre- 
pared the latter by the action of dilute potassium hydroxide on a 
basic cupric nitrate, and Bottger,? by the action of concentrated 
sodium hydroxide on crystalline basic cupric sulfate. Péligot? 
hydrolyzed crystalline blue-violet copper ammonium nitrate; 
while Villiers* claimed that a crystalline hydrate was formed 
from the amorphous hydrous oxide by suspending the latter in 
water which was subsequently frozen and allowed to stand 
several hours. Villiers’ observation supports van Bemmelen’s 
view that the amorphous hydrous oxide goes over gradually to a 
crystalline monohydrate on standing. While the transformation 
of the oxide from the amorphous to the crystalline state on 
standing is an established fact, there is a difference in opinion as 
to whether the crystals are monohydrate. By electrolyzing a 
solution of alkali nitrate,» one obtains blue hydrous copper 
oxide, the physical character and hence the properties of which 
depend on the concentration of solution and the current density.°® 
Miller and Spitzer’ electrolyzed an alkaline copper ammonium 
salt solution with a platinum anode and obtained a black deposit 
containing 95 per cent CuO. In the latter case, dehydration took 
place at the same time as the precipitation. The blue oxide did 
not darken appreciably when suspended for an hour in alkali; 
but if a current was passed through the solution, the particles 
moved to the anode, where partial dehydration and darkening 


1 Compt. rend., 34, 573 (1852); cf. VAN BEMMELEN: Z. anorg. Chem., 5, 
468 (1894). 

2 Jahresber., 198 (1858); HaBERMANN: Z. anorg. Chem., 50, 318 (1906). 

3’Compt. rend., 53, 209 (1861); cf. BonsporFr: Z. anorg. Chem., 41, 132 
(1904). 

4 Compt. rend., 120, 322 (1895). 

5 LorRENZ: Z. anorg. Chem., 12, 438 (1896); Exss: Z. angew. Chem., 17, 
291 (1903). 

6 KoHLSCHUTTER and TiiscHerR: Z. anorg. Chem., 111, 193 (1920); Kout- 
SCHUTTER and SEDELINOvVICH: Z. Elektrochem., 29, 30 (1923). 

7 Kolloid-Z., 1, 44 (1906); Z. anorg. Chem., 50, 322 (1906). 


136 THE HYDROUS OXIDES 


took place by electrical endosmose. Similar observations were 
made with the blue crystalline oxide but the dehydration was 
considerably slower. Miiller and Spitzer believe that definite 
hydrate water would not be removed by electrical endosmose and 
suggest that the chemical hydrate, so called, goes over to an 
unstable adsorption compound or, that an unstable peroxide 
results at first which later decomposes to the ordinary oxide 
containing less water. A more plausible guess is that the crystal- 
line compound is not a definite monohydrate at all but a hydrous 
oxide possessing a dense structure that retains adsorbed water 
more tenaciously than a gelatinous mass.! Since the vapor — 
pressure of hydrous copper oxide becomes practically zero at a 
composition closely approximating CuO-H.O, it cannot be 
determined by vapor-pressure measurements whether a definite 
hydrate exists. This could probably be decided by comparing 
x-radiograms of the black oxide and the blue crystalline com- | 
pound. Hedvall? made such a comparison of black oxides 
prepared in a variety of ways and found them to be identical. 
He also obtained an x-radiogram of the blue compound, but no 
comparison of the latter with the black oxide was recorded. 
Bancroft® points out that, if a definite compound, CuO: H.O, 
exists with a practically zero vapor pressure, it should form from 
cupric oxide in the presence of water; but the reverse process 
is the one that actually takes place. Kohlschiitter and Tiischer+ 
get around this by assuming that the dehydration is not simply 
a molecular splitting off of water, thus: Cu(OH)e = CuO + 
H.O; but depends on intramolecular neutralization of the H° 
and OH’ ions resulting from amphoteric dissociation of Cu(OH). 
as follows: 

Cu(OH)e = Cu” + 20H’ 

Cu(OH)e2 = CuOe” + 2H” 

OH) Be eG 

CuO.” + Cu = 2Cu0 
This interpretation of the mechanism of the dehydration process 
is superfluous, if the blue compound having zero vapor pressure 

1 KoHLSCHUTTER and SEDELINOvICH: Z. Elektrochem., 29, 30 (1923). 
2Z. anorg. Chem., 120, 327 (1922). 


3 “¢ Applied Colloid Chemistry,’’ 246 (1921). 
4Z. anorg. Chem., 111, 193 (1920). 


COPPER, COBALT, NICKEL, SILVER, AND GOLD 137 


is not Cu(OH)s, and may be open to serious question even should 
a monohydrate exist. 

Stability of Blue Cupric Oxide.—The instability of gelatinous 
cupric oxide is one of its most characteristic properties. If 
allowed to stand in contact with its mother liquor, it loses water 
and changes in color from blue to green, brown, and finally 
black,! the process going on slowly at room temperature and 
with increasing velocity as the temperature is raised.” To obtain 
a clear blue product, precipitation should be carried out at 0° 
and the mother liquor removed by washing with iced water as 
rapidly as possible. Analogous to the hydrous oxides of iron 
and chromium, the blue oxide aged at low temperatures holds on 
to its water more strongly than the newly formed product. 

Unlike the gelatinous oxide, the blue crystalline compound is 
stable in boiling water and maintains its blue color when heated 
for hours at 100°. However, it darkens gradually on long stand- 
ing even at room temperature. Thus, 10-year-old samples made 
by the methods of Béttger and Péligot were reported by Fowles* 
to have a bluish-slate tint, while a 12-year-old sample was quite 
black. In the light of these observations Fowles concludes that 
the crystalline oxide is a highly stable form of the blue oxide in a 
state of suspended transformation. 

While alkalies and certain salts tend to decrease the stability 
of the blue gelatinous oxide, Tommasi® found a retardation of 
the blackening in the presence of a number of salts, notably 
MnSO,. Bancroft® attributed the stability to adsorption of 
the hydrous oxide of manganese which acts as a protective col- 
loid. Although this conclusion was reaffirmed by Blucher and 
Farnau’ as a result of observations with salts of a number of 
heavy metals other than manganese, it seems questionable, since 
(1) relatively high concentrations of colloidal hydrous oxides are 


1 ScHaFFNER: Liebig’s Ann. Chem., 61, 168 (1844); Harms: Arch. Pharm., 
(2) 89, 35 (1857); Bull. soc. chim., (2) 87, 197 (1882). 

2Sprine and Lucion: Z. anorg. Chem., 2, 195 (1892); vaN BEMMELEN: 
Ibid., 5, 468 (1894); Euter and EvieEr: Z. anorg. Chem., 124, 70 (1922). 

3 VituiERS: Compt. rend., 120, 322 (1895). 

4 Chem. News, 128, 2 (1924). 

5 Bull. soc. chim., (2) 37, 197 (1882); Compt. rend., 99, 38 (1884). 

6 J. Phys. Chem., 18, 118 (1914). 

7J. Phys. Chem., 18, 629 (1914). 


138 THE HYDROUS OXIDES 


not effective, and (2) copper sulfate is as effective as manganous 
sulfate or chromic sulfate. The first experiment is not especially 
impressive, since there is no necessary reason why the adsorp- 
tion of a colloidal oxide by the separately precipitated copper 
oxide gel should give the same result as adsorption of the oxide 
from a salt solution; but the second observation is fairly con- 
clusive, since it is inconceivable that blue hydrous cupric oxide 
should stabilize itself. Inasmuch as the only salts effective in 
low concentration are those which give an acid reaction by 
hydrolysis there still remains the possibility that the stabilizing 
agent is a basic salt? or an adsorption complex. 

Since there is a marked change in the physical character of 
hydrous cupric oxide during heating in the presence of salts which 
hydrolyze to give an acid reaction, I postulated! a slight solvent 
action which was supposed to destroy the gelatinous structure, 
giving a denser modification that loses water and darkens less 
readily than a loose voluminous mass. Fowles? accepts the 
essential part of this view, that the stability is a result of change in 
‘the physical character of the hydrous oxide; but he very properly 
rules out any solvent action as an important factor in the process. 
Instead, he believes the heavy metal salts remove adsorbed alkali‘ 
as basic salts; thus, the stabilization consists in removing alkali, 
a catalyzer, and allowing the unstable gelatinous substance to 
pass to the more stable crystalline form. Fowles’ hypothesis is 
not particularly helpful or constructive, since it does not attempt 
to define the nature of the alleged catalytic action of alkali on 
the dehydration process. vs 

Like bases, certain alkali salts increase rather than retard the 
spontaneous dehydration of hydrous cupric oxide. Since the 
effect is not appreciable except with relatively high concentra- 
tions of the salts, it probably results from their dehydrating 
action.» An alternative hypothesis is suggested by the behavior 
of hydrous cupric oxide toward alkali salts. Tommasi® found 


1 Weiser: J. Phys. Chem., 27, 501 (1923). 

2 Krier: J. prakt. Chem., 108, 278 (1924). 

3 Chem. News, 128, 5 (1924). 

4 Jorpis: Z. Hlektrochem., 18, 553 (1912). 

5 Poma and Patront: Z. physik. Chem., 87, 196 (1914). 
6 Bull. soc. chim., (2) 37, 197 (1882). 


a 


COPPER, COBALT, NICKEL, SILVER, AND GOLD 139 


that solutions of sodium chloride and sodium sulfate show an 
alkaline reaction after shaking with hydrous cupric oxide. This 
is due to hydrolysis of the salts owing to stronger adsorption of 
acid than of base. The slight decrease in stability in the pres- 
ence of alkali salts may be due to the slight alkalinity of the 
solution in which the particles are suspended. 

Recently, hydrogen peroxide has been found by Quartaroli! 
to accelerate the darkening of hydrous cupric oxide suspended 
in a definite amount of alkali at 50°. This action is still per- 
ceptible with 1 part of peroxide in 200 million of water. In 
view of the presence of minute traces of hydrogen peroxide 
in ordinary distilled water, this compound is believed to bring 
about the spontaneous decomposition of the hydrous oxide or, 
at least, to accelerate the process. Such sensitive action of 
extremely minute amounts of substance has been found only in 
the action of copper in provoking the oxidation of sulfites and 
in the quantity of substance required to break up metastable 
states. Various electrolytes, especially magnesium salts, retard 
the blackening when present in amounts hundreds of times less 
than that of the hydrous oxide; but the action of such electrolytes 
exhibits striking irregularities. Quartaroli concludes that the 
blackening of the oxide suspended in alkali solution is not a 
simple dehydration process but is a phenomenon connected with 
oxidations and reductions with the formation of saline hydrates 
containing copper atoms with various grades of oxidation. This 
conclusion is so hopelessly vague and indefinite that the obser- 
vations should be confirmed and extended. 

By dehydrating the blue oxides under suitable conditions, 
compositions approximating the formulas for hydrates have been 
obtained.2 Recently, Losana* obtained temperature-composi- 
tion, vapor-pressure, and electromotive-force curves indicating 
the formation of hydrates in which the ratios CuO: H:20 are 3, 
4, and 8:1 when the dehydration takes place in the presence of 
liquid and 3, 4, 6, 7, and 8:1 when the compound has been 
dried before dehydrating. The presence of alkali and other 


1 Gazz. chim. ital., 55, 264 (1925). 

2See Mertuor: ‘Treatise on Inorganic and Theoretical Chemistry,’’ 3, 
142 (1923). 

3 Gazz. chim. ital., 58, 75 (1923). 


140 THE HYDROUS OXIDES 


salts influences the dehydration temperature, and in some 
instances, loss of water occurs below what was regarded as the 
true inversion point. Such behavior is not characteristic of 
well-defined stable hydrates. 

Color.—As already noted, hydrous copper oxide may be blue, 
green, olive, brown, or black in color. The change in color is 
not necessarily associated with loss of water, as De Forcrand! 
observed a change in Peligot’s oxide at 85° from blue to green 
without loss of weight. De Forcrand dissolved the blue, green, 
olive, and brown hydrous oxides and the black anhydrous oxide 
in sufficient nitric acid to form the nitrate; and Sabatier? and 
Joannis® carried out similar experiments on the oxides dehydrated 
at 440° and atred heat. From thesedata De Forcrand concludes 
that the different colored oxides are isomers involving definite 
heat changes in the transformation from one form to another. 
It would seem, however, that thermochemical evidence of this 
sort is altogether insufficient to establish the existence of definite 
isomers. Many hydrous oxides, when heated quickly, undergo 
a decrease in surface energy which is sufficient to raise the tem- 
perature of the whole mass to incandescence. Thermochemical 
data obtained before and after glowing might be interpreted 
to mean that an isomeric transformation had taken place; but 
such a conclusion would be erroneous. It is much more probable 
that the differences in color result from differences in the physical 
character and size of particles. Kohlschiitter and Tischer 
believe the blue compound to be amorphous or pseudocrystalline 
Cu(OH)., consisting of rather large particles, and the black 
compound to be CuO made up of distinctly smaller particles. 
The conclusion that the blue particles are larger than the black 
seems to have been reached without taking all the facts into 
consideration. Everyone knows that copper oxide can be 
obtained in quite large particles which are black and the hydrous 
oxide in very much more highly dispersed, gelatinous particles 
which are blue. A clump of the blue hydrous oxide consists of 
very finely divided primary particles that have adsorbed water 
strongly, forming a gelatinous mass. On heating, the relatively 

1 Compt. rend., 157, 441 (1913). 


2 Compt. rend., 125, 301 (1897). 
3 Compt. rend., 102, 1161 (1886). 


COPPER, COBALT, NICKEL, SILVER, AND GOLD 141 


large gelatinous clump is broken up and the particles constituting 
it coalesce to larger particles that appear black. From this point 
of view, anhydrous cupric oxide would be blue and not black if 
coalescence during dehydration were prevented. In support of 
this, Schenck,! in Bancroft’s laboratory, observed that a mixture 
of the hydrous oxides of copper and aluminum containing 5 
per cent cupric oxide remained blue after ignition. In one 
instance” the excess of alumina was dissolved out with alkali, 
giving a distinctly blue powder containing CuO and Al,O3 in 
the ratio of approximately 4:1. 

The hypothesis of Kohlschiitter that the blue and black oxides 
are Cu(OH)2. and CuO, respectively, stands or falls on the 
unproved assumption that there is a definite amorphous hydrate 
and that this dehydrates not by the molecular splitting off of 
water but by “internal neutralization as a result of amphoteric 
dissociation.” 

The wide variation in color of anhydrous CuO is utilized in 
coloring glass and pottery glazes. That cupric oxide imparts a 
-blue or green color to glass under certain conditions was known to 
the ancients and the later alchemists. An old blue Venetian 
glass contained 1.382 per cent CuO.* Artificial emeralds have 
been prepared with this pigment. ‘The color imparted to a glaze 
by CuO depends on the constituents of the glaze and the condi- 
tions of firing. Ina reducing atmosphere, a red glaze is obtained 
consisting probably of colloidal copper or cuprous oxide; blue and 
green glazes develop in an oxidizing atmosphere.* 

Cupric Oxide Sols.—To prepare sols of hydrous cupric oxide, 
it is usually necessary to employ some protective agent. Gra- 
ham? prepared a fairly stable sol by adding alkali to a solution of 
cupric chloride containing cane sugar. It was deep blue at first 
but changed to green on dialyzing; the precipitate obtained on 
boiling the sol or on adding salts or acids was bluish green in 
color. A more stable sol can be prepared using Paal’s® sodium 


1 J. Phys. Chem., 23, 283 (1919). 
2 PARSELL: J. Phys. Chem., 26, 501 (1922). 

3 Scuwarz: Dinglers polytech. J., 205, 425 (1872). 
4RavuTER: Z. angew. Chem., 14, 753 (1901). 
5 Phil. Trans., 161, 183 (1861); Compt. rend., 59, 174 (1864). 
6 Ber., 39, 1550 (1906); Kolloid-Z., 30, 1 (1922). 


142 THE HYDROUS OXIDES 


salts of lysalbinic or protalbinic acids as protective colloid. Other 
stabilizing agents that have proved effective are agar,! casein,’ 
milk and grape sugar,* and soap. Thorium and uranyl nitrate* 
peptize the gelatinous oxide owing to strong adsorption of the 
salt cations. Biltz> attempted to prepare the colloidal oxide by 
dialysis of a solution of cupric nitrate; but the salt passed 
unchanged through the dialyzer owing to its low hydrolysis 
constant. Ley® hydrolyzed the copper salt of succinimide, 
obtaining a very satisfactory sol that changed in color, slowly 
at room temperature but rapidly at 70°, from blue green to 
yellow brown and finally dark brown. Succinimide is a 
protector, since its removal by dialysis causes agglomeration of 
the sol. ! 

Cupric oxide sols have been prepared without the use of a 
protective colloid by oxidizing a copper sol’ and by what Kohl- 
schiitter called ‘discharge electrolysis.”’® In the latter process, 
a passivifying layer of oxide was deposited on the anode and sub- 
sequently dispersed in the liquid by a rapidly oscillating discharge. 
The sol was perfectly clear in transmitted light; it was bluish: 
green in color at the outset but changed to brown on standing. 
By means of the ultramicroscope, Kohlschiitter® observed the 
formation of blue hydrous cupric oxide sol when a current of 
relatively high density was passed between copper electrodes 
dipped in dilute CuSO, solution or in water. 

By passing a spark between copper wires under water, there is 
formed a positive sol of cupric oxide instead of copper.’® Stirring 
accelerates the velocity of coagulation of this sol by electrolytes, 
particularly when ions of high coagulating power are employed 
and when the concentration of electrolyte is in the region of the 


1Lupwic: Brandes Archiv, 82, 157 (1855). 

2 RITTENHAUSEN: J. prakt. Chem., [2] 5, 215 (1873); 7, 361 (1874). 

3 Saukowsky: Pfliiger’s Arch., 6, 221 (1872); Sen and Duar: Kolloid-Z., 
38, 193 (1923). 

4 SztLarD: J. chim. phys., 5, 636 (1907). 

5 Ber., 35, 4431 (1902). 

6 Ber., 38, 2199 (1906); Ley and WernzER: Jbid., 39, 2178 (1906). 

7 LoTTERMOSER: “ Anorganische Kolloide,’”’ Stuttgart (1901). 

8 KOHLSCHUTTER: Z. Elektrochem., 25, 309 (1919). 

9Z. Elektrochem., 30, 164 (1924). 

10 Burton: Phil. Mag., [6] 11, 436 (1906). 


COPPER, COBALT, NICKEL, SILVER, AND GOLD 148 


precipitation value. Prolonged stirring alone, without the addi- 
tion of precipitating electrolyte, lowers the charge on the par- 
ticles sufficiently to allow agglomeration and finally complete 
precipitation. ! 

Hooker prepared a very satisfactory blue sol? by thorough 
washing of the gelatinous oxide formed by adding alkali to copper 
sulfate solution until the supernatant liquid was just colorless. 
The washing may be carried out by sedimentation, but a more 
stable sol results by repeated washing with the supercentrifuge.* 
The colloidal oxide has high fungicidal action against apple scab 
and apple black in concentrations of 1 part of hydrous oxides to 
5000 of water; at this concentration it causes very slight burning. 
This sol possesses excellent sticking properties due to its positive 
charge and can be used in conjunction with lead arsenate and 
nicotine sulfate, if desired. Since the sol can be prepared at 
relatively low cost, it is suggested as a substitute for Bordeaux 
mixture and lime sulfur. 

Hydrous cupric oxide dissolves but slightly in dilute alkali, but 
is appreciably soluble in concentrated alkali forming deep-blue 
solutions.4 As might be expected, the unstable blue gelatinous 
oxide is more soluble than the black compound, and the latter 
separates gradually from a solution of the former, provided the 
alkali (NaOH) concentration does not exceed 17 NV. Both cop- 
per, in the presence of air, and anhydrous CuO dissolve in alka- 
lies, forming blue solutions which are stable on boiling and do not 
precipitate out on standing.® The bulk of the evidence supports 
the view that the blue coloration is due to CuO,’”” ion and not to 
colloidal cupric oxide. Fischer® added alkali to copper salts 
short of precipitation and obtained blue solutions which were 
supposed to be colloidal, because hydrous copper oxide settled 


1 FREUNDLICH and Basu: Z. physik. Chem., 115, 203 (1925). 

2J. Ind. Eng. Chem., 15, 1177 (1923). 

3 BRADFIELD: J. Am. Chem. Soc., 44, 965 (1922). 

4 Low: Z. anal. Chem., 9, 463 (1870); Donatu: [bid., 40, 137 (1901). 

5 MULurR: Z. physik. Chem., 105, 73 (1923). 

6 CREIGHTON: J. Am. Chem. Soc., 45, 1237 (1923). 

7MeELBye: Meddel. Vetenskapsakad. Nobelinst., 4, 8 (1922); Chem. 
Abstracts, 17, 1572 (1923). 

8 Z. anorg. Chem., 40, 39 (1904); CHATTERJI and Duar: Chem. News, 121, 
253 (1924). 


144 THE HYDROUS OXIDES 


out on standing or could be filtered out. The discoloration of 
the solution by filtering was believed by Creighton to result from 
interaction between the cellulose of the filter and the blue com- 
ponent, since the filtrate was blue after several portions were 
passed through the same filter. This observation is not quite 
conclusive, since the filter may have become more porous owing 
to peptization of the filter paper by the alkali. However, 
Miller prepared a cobalt-blue crystalline cuprite from the alkali 
solution; so there is no doubt of the existence of such a salt. It 
is altogether probable that sol formation precedes cuprite forma- 
tion when alkali acts on the gelatinous oxide. 

The solubility of hydrous cupric oxide in alkali may be 
increased enormously by the presence of other substances. Thus, 
the addition of alkali to copper sulfate in the presence of tartrate 
forms the deep-blue solution known as Fehling’s solution, so widely 
used in detecting small amounts of reducing sugar. ‘The copper 
in this solution is not present as cupric oxide sol but as a cupric 
tartrate complex. The same is apparently true for the alkali 
solution formed in the presence of higher valent alcohols, such 
as glycerin and mannite, and of certain amines.” Gelatinous 
copper oxide is adsorbed strongly by hydrous chromic oxide, 
and the colloidal solution of the latter in alkali carries a consider- 
able amount of the former into colloidal solution.* 

Cupric Oxide Jellies.—A cupric oxide jelly forms on hydrolysis 
of a solution of cupric ammonium acetate of the formula Cu- 
(CeH302)2-2NH3.4 It is unnecessary to start with this salt, 
and much more stable jellies are obtained by precipitation of a 
suitable amount of colloidal oxide at a suitable rate. The 
desired conditions may be realized® by adding ammonia to cupric 
acetate in the presence of a small amount of sulfate® and allowing 
the instable colloidal solution to precipitate spontaneously. 


1 Kuster: Z. Elektrochem., 4, 112 (1897); Masson and STEELE: J. Chem. 
Soc., 75, 725 (1899); KAHLENBERG: Z. physik. Chem., 17, 577 (1895). 

2 TRAUBE: Ber., 55, 1899 (1922); 56, 1653 (1923); Donnan: Abegg’s 
‘‘Handbuch anorg. Chem.,’’ 2, 547 (1908). 

3Cf. Knecut, Rawson, and Léwrentua.: ‘A Manual of Dyeing,” 1, 
241 (1916); Prup’HomME: Bull. soc. chim., (2) 17, 253 (1872). 

4 ForrstTEer: Ber., 25, 3416 (1892); Frncu: J. Phys. Chem., 18, 26 (1914). 

5 WEISER: J. Phys. Chem., 27, 685 (1923). 

6 Fincu: J. Phys. Chem., 18, 26 (1914). 


COPPER, COBALT, NICKEL, SILVER, AND GOLD 145 


The solution obtained is perfectly clear at the outset, but pre- 
cipitation starts after intervals varying from a few seconds to 
several minutes depending on the relative amounts of the three 
components. In view of the great importance of rate of pre- 
cipitation on jelly formation, the most favorable conditions are 
pretty sharply defined. A firm jelly that remained unbroken 
for weeks is obtained by mixing 5 cubic centimeters of 3 N 
NH,OH to 25 cubic centimeters of 0.75 N Cu(C2H302)2 con- 
taining 2 cubic centimeters of N K,SO,. The presence of sulfate 
is necessary in order to get a sol of sufficient concentration. 
Gelatinous precipitates instead of jellies are obtained by adding 
ammonia directly to copper sulfate, chloride, or nitrate, on 
account of the high velocity of precipitation. 


Hyprovus Cuprous OXIDE 


The yellow or orange precipitate thrown down from a cuprous 
salt solution by sodium hydroxide! or carbonate? is not CuOH 
but hydrous cuprous oxide in an amorphous state.’ The yellow 
compound is best prepared by reduction of Cu’’ in the presence 
of OH’ or by electrolysis of alkali salts in the cold with a copper 
anode.* For the chemical reduction, a variety of reducing agents 
may be used, such as dextrose,° maltose,* and phenylhydrazine ;’ 
hydroxylamine hydrochloride® is particularly satisfactory. The 
yellow oxide is formed in the cold by the reduction of Fehling’s 
solution with dextrose or in the hot when the amount of tartrate 
is too small to convert all the Cu’ into a complex. The clear- 
yellow amorphous product goes over rapidly to orange or reddish 
yellow, probably with the loss of water. By drying in the 
absence of air, a stable product is obtained. This is changed 


1 MirTscHERLICH: J. prakt. Chem., 19, 450 (1840); Proust: J. phys., 61, 
182 (1800); Kuason: Svensk Kem. Tid., 36, 202 (1924). 

2FremMy: Ann. chim. phys., [3] 28, 391 (1848). j 

3 GROGER: Z. anorg. Chem., 81, 326 (1902); Mosmr: /bid., 105, 112 (1919). 

4 LORENZ: Z. anorg. Chem., 12, 488 (1898). 

5 SANDMEYER: Ber., 20, 1494 (1887); Miniter and Haaren: Pfltiger’s 
Arch., 28, 221 (1880). 

6 GLENDENNING: J. Chem. Soc., 67, 999 (1895). 

7 CHATTAWAY: Chem. News, 97, 19 (1908). 

8 Moser: Z. anorg. Chem., 105, 112 (1919); cf. Epuer: Ber., 35, 3055 
(1902). 


146 THE HYDROUS OXIDES 


to a brick-red amorphous powder by heating for 60 hours at 
500°; and by igniting in a stream of nitrogen, it goes over to the 
familiar crystalline red form which is commonly obtained by 
reducing Fehling’s solution at 100°. Similarly, by electrolyzing 
alkali salts, the oxide is yellow at room temperature, bright red 
at 100°, and intermediate colors in between.! Red cuprous 
oxide is prepared on a commercial scale by electrolysis of a hot 
solution of sodium chloride with copper anodes. 

Red crystalline Cuz,O appears to bear the same relation to the 
yellow hydrous oxide that black CuO bears to the blue gelatinous 
oxide. The change in the color of hydrous cuprous oxide from 
yellow, through orange, brick red, and bright red results from a 
gradual increase in particle size and loss of absorbed water. 
With this change from the finely divided hydrous precipitate 
having a high specific surface to the granular red product having 
a low specific surface, the reactivity with oxygen and the solu- 
bility in acids and alkalies fall off. The varicolored products are 
not definite isomeric modifications of Cu20O. 

The colorless solution of hydrous cuprous oxide in ammonia 
turns blue in the air owing to the formation of Cu(NHs).’’, 
thereby furnishing a delicate test for oxygen. 

Red cuprous oxide is largely used in coloring glass red, a property 
known to the ancients and in the middle ages. The manufacture 
of this ancient red glass was revived by Bontemps in France and 
Englehardt in Germany about 1827. The oxide is also used for 
the production of a red glaze on pottery? and as a poisonous 
pigment in antifouling compositions for painting the bottoms of 
vessels. 

Cuprous Oxide Sols.—A sol of hydrous cuprous oxide is nearly 
always obtained in the reduction of alkali solutions of copper 
salts. * Similarly, during the reduction of cupric sulfate solution 
with. SnCle* or hydrazine, yellow hydrous® cuprous oxide first 
appears, which goes over to the red oxide and finally to black 

1 Mitumr: J. Phys. Chem., 18, 256 (1909); Moser: Z. anorg. Chem., 
105, 112 (1919). : 

*Louts and Dutatuity: Mon. Ceram. et Verr., 19, 237. 

3 Cf. Ruoss: Z. anal. Chem., 68, 193 (1919). 

4 LOTTERMOSER: J. prakt. Chem., (2) 59, 492 (1899). 


5GuTBIER and HormeierR: Z. anorg. Chem., 32, 355 (1902); 44, 227 
(1905); cf. PAau and Lruze: Ber., 39, 1550 (1906). 


COPPER, COBALT, NICKEL, SILVER, AND GOLD 147 


copper sol. Reduction of a neutral solution of copper sulfate in 
the presence of gum arabic or gelatin! gives a cuprous oxide sol 
which is stable in the absence of air. Grdéger? obtained a stable 
sol on attempting to purify the orange hydrous oxide by pro- 
longed washing in the absence of air. 


Hyprovus CoBALTOUS OXIDE 


If alkali is added in excess to a solution of rose-colored cobalt- 
ous salt, there is formed, at first, a blue hydrous mass which goes 
over into rose hydrous cobalt oxide or hydroxide, the transforma- 
tion taking place more rapidly the higher the temperature.? 
The gradual transformation can be observed by boiling cobaltous 
carbonate with a solution of potassium hydroxide. The volu- 
minous blue oxide formed at first turns to violet and then to 
rose red.* On the other hand, if insufficient akali to react with 
all the cobalt ion is used, the precipitate retains its blue color 
indefinitely. At one time, the blue compound was believed to 
be a basic salt which was decomposed by excess alkali, forming 
rose-colored hydrate. Hantzsch* disproved this view by showing 
that the blue oxide, precipitated from sulfate or acetate solution 
with insufficient alkali, absorbs the respective salts or basic 
salts strongly; but most of the latter can be removed by repeated 
boiling of the precipitate with water free from air, without alter- 
ing the blue color in any way. Hence, the blue color is that 
of the oxide and not of a basic salt. 

The blue and rose precipitates differ quite appreciably in their 
chemical properties. Thus, the blue compound loses practically 
all its water at 150° and is completely dehydrated at 170°; 
whereas the rose compound still retains some water after heating 
for several hours at 300°. Moreover, the blue oxide reacts 
slowly with acetyl chloride, while the red reacts so rapidly that 
the chloride boils. After drying the preparations in a desiccator, 
each analyzes experimentally for a monohydrate or hydroxide. 


1 Lopry Dr Bruyn: Rec. trav. chim., 19, 236 (1900). 

2Z. anorg. Chem., 31, 326 (1902). 

3 WINKELBLECH: Liebig’s Ann. Chem., 18, 148 (1835); Brrz: Pogg. Ann., 
61, 472 (1844). 

4Fremy: Jahresber., 637 (1851). 

5Z. anorg. Chem., 78, 304 (1911). 


148 THE HYDROUS OXIDES 


Hantzsch concludes, therefore, that the two compounds are 
hydrate isomers differing in the way in which the water is held, 


the blue being CoO: H.20 and the red Co __ ae 


As has been pointed out, the gelatinous oxide does not go over 
to rose in the absence of alkali or in the presence of a little cobalt 
salt. Benedict! observed that the change in color from blue to 
rose in the presence of excess alkali is retarded by the addition 
of a small amount of nickel salt. The retardation is sufficiently 
marked to serve as a delicate test for nickel. To account for 
this behavior, Benedict postulates the formation of a deep-blue 
nickel cobaltite which masks the rose-colored hydrate. This 
assumption cannot be correct, since increasing the amount of 
nickel salt does not increase the intensity of the blue color. 
Apparently, hydrous nickel oxide is adsorbed by the blue oxide, 
thereby stabilizing it to a certain extent. The phenomenon 
recalls the stabilization of blue hydrous cupric oxide by salts, but 
it differs from the latter in that salts of metals other than nickel 
have little or no effect. Thus the presence of the sulfates of 
iron(ous), zinc, manganese, magnesium, chromium, copper, 
and aluminum; the chlorides:of tin and calcium; and the nitrates 
of lead, cadmium, thorium, and strontium produces no marked 
retardation.” The specific effect of nickel oxide may be due to 
its having the same crystal lattice as cobalt oxide. 

Both the blue and rose compounds thrown down by mixing 
solutions of cobalt salt and alkali are gelatinous and appear 
amorphous. It is possible that both are hydrous oxides when 
first formed but, like cadmium oxide, go over into microcrystal- 
line hydrates on standing. This is certainly true of the rose 
compound, as shown by x-ray examination.* Large crystals of 
rose hydroxide come down from a solution made by adding 250 
grams of potassium hydroxide to 10 grams of cobalt chloride 
hexahydrate in 60 cubic centimeters of water, and heating in 
the absence of air until solution is complete. The crystals are 
elongated rhombic prisms that analyze for Co(OH):.4 They are 


1 J. Am. Chem. Soc., 26, 695 (1904). 

2 CHATTERJI and Duar: Chem. News, 121, 253 (1920). 
3 HEDVALL: Z. anorg. Chem., 120, 327, 338 (1924). 

4 Dr SCHULTEN: Compt. rend., 109, 266 (1889). 


COPPER, COBALT, NICKEL, SILVER, AND GOLD 149 


pleochroic, appearing rose colored along n,, rose yellow along 
Nm, and pale brownish yellow along n,. Unlike the gelatinous 
oxide, the large crystals are not altered by contact with air. 

Since a crystalline rose-colored oxide is known with certainty, 
and there is only an analysis of an apparently amorphous mass to 
indicate the nature of the blue preparation, it might be concluded 
that the blue compound is a hydrous. oxide and the rose, a 
hydroxide. ‘This hypothesis, like that of Hantzsch’s, cannot be 
correct, since it is based on the manner in which water is held 
by the oxide, and apparently we may have either a blue or a rose 
oxide in the absence of water. Thus cobalt glass owes its color 
to the blue anhydrous oxide; and the brown anhydrous powder 
obtained by drying the precipitated hydrate melts without 
decomposition and gives rose-colored crystals on cooling.t This 
suggests the possibility of the color of cobalt oxide and hydroxide 
being determined by the size of particles. In glass the particles 
are obviously highly dispersed and appear blue, while the oxide 
in mass is red. Similarly, the precipitated oxide is most finely 
divided when first formed and so is blue; but in the presence of a 
slight excess of alkali, the highly hydrous mass ages, losing water 
and becoming denser, the color at the same time changing from 
blue through lavender to rose. The rate of this transformation is, 
of course, hastened by raising the temperature, and is retarded 
or stopped by the presence of basic cobalt salts or hydrous 
nickel oxide. 

If one objects to attributing the difference in color to the size of 
particles, an alternative hypothesis is that there are two allo- 
tropic forms of cobalt oxide, an instable blue oné and a stable 
rose. As a matter of fact, the transformation of the blue gelati- 
nous oxide to rose in the presence of alkali has led people to regard 
the former as the alkali instable modification and the latter as 
the alkali stable modification.? The existence or non-existence 
of allotropic forms could probably be settled by comparing 
x-radiograms of the blue and rose oxides or hydrates.* 


1 Morssan: Ann. chim. phys., (7), 4, 186 (1895); Hepvauu: Z. anorg. 
Chem., 86, 210 (1914); Hantzscu: Z. anorg. Chem., 73, 304 (1912). 

2 HantzscH: Z. anorg. Chem., 73, 304 (1912); Farnav and WITTEVEEN: 
J. Ind. Eng. Chem., 18, 1060 (1921). 

3 Cf. HEDVALL: Z. anorg. Chem., 120, 338 (1922). 


150 THE HYDROUS OXIDES 


Both blue and rose cobalt oxide dissolve in concentrated alkali, 
giving a solution with a deep-blue color. A similar color results 
on electrolyzing a solution of alkali, 4 N or stronger, with a cobalt 
anode. The blue solution was thought by Tubandt! to be 
colloidal cobalt oxide; but the results of exact potential measure- 
ments of cobalt against the blue solutions containing different 
amounts of cobalt in 8 N potassium hydroxide show conclusively 
that the blue color is due to potassium cobaltite, KeCoOzs, and 
not to colloidal cobalt oxide.2 Thus the behavior of cobalt oxide 
in excess alkali is similar to that of cupric oxide. In the light of 
these observations, it is unlikely that the alkaline solution in 
the presence of glycerin is colloidal.* Positive sols have been 
formed both by peptization of the blue oxide with dilute hydro- 
chloric acid? and by thorough washing of the fresh gelatinous 
precipitate;> but they are quite instable, settling out in the 
course of a few hours. 

Liesegang rings of Co(OH)> are formed by pouring ammonia on 
a gelatin gel containing cobalt chloride. Under certain condi- 
tions a spiral is obtained instead of a series of rhythmic bands.°® 

Cobaltous oxide is used by enamelers and porcelain manu- 
facturers for the production of the finest blue glaze and color in 
porcelain, glass, and other vitrifiable substances; 1 part of oxide 
in 100,000 imparts a faint blue while 1 part in 1000 gives a deep 
blue. When heated with certain oxides, colored compounds 
are formed, and with others, colored solid solutions which are 
widely used as pigments. With aluminum oxide, a blue com- 
pound CoO-Al.03 known as cobalt blue or Thenard’s blue is 
formed at 1100°, and above this temperature a green one, said 
to have the composition 4CoO:3AI,03.7 The exact tint of 
cobalt blue depends on the conditions of formation and on the 
relative amounts of the two oxides fused. A similar valuable 


1Z. anorg. Chem., 45, 368 (1905); cf. Donatu: Monatschefte fiir Chemie, 
14, 93 (1893). 

2 GRuBE and Frucnut: Z. Elektrochem., 28, 568 (1922). 

3 Cf., however, SEN and Duar: Kolloid-Z., 33, 193 (1923). 

4 MUuuer: Z. anorg. Chem., 57, 311 (1908). 

5 Tower and Cooke: J. Phys. Chem., 26, 733 (1922). 

6 WoLFGANG OsTWALD: Kolloid-Z. (Zsigmondy Festschrift), 36, 390 (1925). 

7 HEeDvALL: Archiv Kemi, Mineral. Geol., 5, 18, 1 (1914); Z. anorg. Chem., 
96, 71 (1916). 


COPPER, COBALT, NICKEL, SILVER, AND GOLD 151 


pigment known as cobalt green or Renneman’s green is obtained 
by fusing cobalt oxide with zinc oxide. The green color is due 
to cobalt zincate which forms solid solutions with excess zinc 
oxide.t Stannic oxide likewise forms a green stannate,? and 
chromic oxide, a green chromite*® on fusion with cobalt oxide. 
The chromite dissolves in excess of either oxide, giving various 
shades of blue. Many combinations with other oxides have 
been reported, but in the majority of cases these are either 
mixtures or solid solutions. Magnesium oxide forms mixed 
crystals varying in color from light to dark red, depending on 
the relative proportions of the two oxides. Mixed erystals are 
also formed with the isomorphous oxides of nickel and manga- 
nese.® It is probable that cobalt oxide is dissolved or dispersed 
by silica although violet cobalt orthosilicate and ruby-red meta- 
silicate have been reported.’ Obviously, the deep-blue color of 
cobalt glass is not due to the formation of these alleged compounds. 

Cobalt oxide proves to be a very good ‘‘dryer”’ for paints.*® 
Since the action of dryers is to catalyze the oxidation of the oil, 
‘this behavior is in line with the observations that the spontaneous 
oxidation of cobaltous hydrate induces the oxidation of the 
stable nickelous hydrate. ® 


Hyprovus CoBALTIC OXIDE 


Cobaltice oxide, Co20s;, in a highly hydrous state is precipitated 
on treating a solution of a cobaltous salt with alkaline hypo- 
chlorite!® or persulfate;!! or by electrolysis of an alkaline solution 


1 HEDVALL: Z. anorg. Chem., 93, 313 (1915); 96, 71 (1916); cf., however, 
[bid., 86, 201 (1914). 

2 HEDVALL: Archiv Kemi, Mineral. Geol., 5, 18, 1 (1914). 

3 BuuioT: ‘On the Magnetic Combinations,’ Gottingen, 33 (1862). 

4Farnavu and WITTEVEEN: J. Ind. Eng. Chem., 13, 1061 (1921). 

5 HEDVALL: Z. anorg. Chem., 86, 296 (1914). 

6 HepvALt: [bid., 92, 381 (1915). 

7RtaerR: Keram. Rundschau, 31, 79, 87, 99, 110 (1923); C. A., 18, 156 
(1924). 

8 GARDNER and Parks: Paint Mfrs.’ Assoc. U. S., Circ. 186 (1923). 

9 Mirrra and Duar: Z. anorg. Chem., 122, 146 (1922). 

10 Carnot: Compt. rend., 108, 610 (1889); ScuR6pER: J. Chem. Soc., 
58, 1213 (1890). 

11 Mawrow: Z. anorg. Chem., 24, 263 (1900); Htrrner: Jbid., 27, 81 
(1901). 


152 THE HYDROUS OXIDES 


of cobalt sulfate.! It forms a brownish-black mass that loses 
water readily, the composition depending on the method of 
drying. The dark-brown anhydrous powder is transformed into 
black cobaltous cobaltic oxide, Co304, corresponding to magnetic 
oxide of iron, by heating below 910°.? 

Hydrous Co30, results when cobaltous hydroxide oxidizes in 
the air. A fairly pure preparation is obtained by warming 
cobaltous hydroxide with an excess of ammonium persulfate, 
washing and heating the product with dilute nitric acid. The 
substance obtained by fusing an oxide of cobalt with caustic 
potash which was thought to be a potassium cobaltite, CogOi6- 
Ke:3H20,* is probable cobalto-cobaltic oxide with adsorbed 
potash.® 


Hyprovus NICKELOUS OXIDE 


The addition of potash or soda to a solution of nickel salt 
throws down a voluminous apple-green precipitate of hydrous 
nickel oxide which goes over to crystalline® Ni(OH):.? The 
purest preparation is obtained by using nickel nitrate or nickel 
ammonium nitrate rather than sulfate or chloride, since nitrate 
ion is said to be least strongly adsorbed by the hydrous precipi- 
tate.* The gelatinous oxide is readily soluble in ammonia form- 
ing a deep-blue solution from which a green crystalline powder 
is precipitated by boiling. 

Anhydrous NiO is an olive-green powder which becomes deep 
yellow on heating but returns to the original green on cooling to 
room temperature.® Like cobalt oxide, it forms a variety of 


1 CorHN and GLAsER: Z. anorg. Chem., 33, 9 (1903). 

2 BURGSTALLER: Chem. Zentr., II, 1525 (1912). 

3 Mawrow: Z. anorg. Chem., 24, 263 (1900). 

4 SCHWARZENBERG: Liebig’s Ann. Chem., 97, 212 (1856); PrsBau: Jbid., 
100, 257 (1856); Mayr: /bid., 101, 266 (1857). 

5 McConne.u and Hanus: J. Chem. Soc., 71, 585 (1897). 

6 HEDVALL: Z. anorg. Chem., 120, 338 (1922). 

7 TowErR: J. Phys. Chem., 28, 176 (1924). 

8 BonsporFF: Z. anorg. Chem., 41, 136 (1904); TrrcumMann: Liebig’s 
Ann. Chem., 156, 17 (1870). 

9 Morssan: Ann. chim. phys., [5] 21, 238 (1880); Zimmerman: Liebig’s 
Ann. Chem., 232, 344 (1880). 


COPPER, COBALT, NICKEL, SILVER, AND GOLD 158 


pigment colors for glazes! when fused with other metallic oxides. 
Thus alumina gives a blue aluminate, and zine oxide, a blue 
zincate; but a variety of tints is possible, as the compounds form 
solid solutions with the excess of either oxide. Mixed crystals 
result on fusing nickel oxide with the isomorphous oxides of 
magnesium, nickel, and cobalt.? 

To prepare the active form of Ni(OH)e for the Edison storage 
battery, the oxide is precipitated from sulfate solution with an 
excess of sodium hydroxide. The gel is dried slowly with the 
- enclosed salts and alkali which are subsequently washed out. 
Excess of alkali increases the porosity of the product.’ 

As early as 1906, Ipatiev+ employed nickel oxides as catalytic 
agents for hydrogenation, working at temperatures around 250° 
and at 100 atmospheres pressure. Later Bedford and Erdmann? 
found nickel oxides to be efficient catalyst for the hydrogenation 
of oils ‘The oxides were considered to be superior to metallic 
nickel: first, because the velocity of hydrogenation is more rapid 
with the former; and second, because the oxides are much less 
sensitive to the action of poisons such as sulfur, chlorine, and 
carbon monoxide. The latter gas is especially poisonous to 
nickel, but in technical hydrogenation with oxides, it can be 
allowed to accumulate in the system without having any effect 
except to dilute the system. Nickelous and nickelic oxides are 
effective at 250°; at temperatures as low as 180°, the most effi- 
cient catalyst appears to be suboxide, possibly NiO, which forms 
colloidal solutions in oil. Indeed, the increased activity of the 
higher oxides, after using for a short time, is attributed to the 
formation of a colloidal solution of nickel suboxide. 

Since nickel oxide is reduced by hydrogen at 190°, it is claimed 
by some that the actual hydrogen carrier in Erdmann’s experi- 
ments was metallic nickel. This does not seem to be the case, 
since the catalyst freed from the hardened oil is a strongly mag- 


1Cf. Wuirner: J. Am. Ceram. Soc., 4, 357 (1921). 

2 HepvaLu: Z. anorg. Chem., 103, 249 (1918). 

3 Epison: U.S. Patents 1083355—-1083356 (1914); 1167484 (1916). 

4 J. Russ. Phys.-Chem. Soc., 38, 75 (1906); 39, 693 (1907); 40, 1 (1908). 

5 J. prakt. Chem., (2) 87, 245 (1913); J. Russ. Phys.-Chem. Soc., 46, 
616 (1913); British Patent 29612 (1910); 18122 (1913). 

6 MEIGIN and Bartg.s; J, prakt. Chem., (2) 89, 290 (1914). 


154 THE HYDROUS OXIDES 


netic black powder which does not form a carbonyl and does not 
conduct the current as does nickel. Moreover, finely divided 
nickel is a hydrogenation catalyst either in the presence or 
absence of moisture, whereas the suboxide is inactive except in 
the presence of moisture.! According to Erdmann, the suboxide 
forms an additive product with the oil which assists in preventing 
reduction to metallic nickel. 

Sabatier and Espel? prepared what they took to be NuO by 
reducing NiO with hydrogen at 220°; but this differs from the 
catalyst, as it forms a carbonyl. Erdmann prepared an oxide 
very similar in properties to that obtained from the hardened 
oil by electrical reduction of potassium nickel cyanide.* ‘The 
product was colloidally dispersed by oil and proved to be a good 
catalyst for hydrogenation. While the evidence points to the 
existence of a catalytically active suboxide of nickel, its composi- 
tion has not been established with certainty.* 

A sol of nickel hydroxide results on mixing solutions containing 
equivalent amounts of nickel tartrate and potassium hydroxide. 
If the solutions are as concentrated as normal, precipitation 
takes place slowly giving a transparent green jelly; but if the 
solutions are dilute, say N/10, a sol forms which can be purified 
by dialysis. 

The gel precipitated, from nickel chloride solution by alkali 
is peptized by washing. Using N/10 solutions, six or seven 
washings by decantation suffice. ‘Tower® attributes the stabili- 
zation to potassium chloride, since sol formation is retarded or 
prevented by washing either too little or too much. One mol 
of KCl to 200 mols of Ni(OH): was found to be the limiting ratio 
for a stable sol.® 


1 SENDERENS and ABOULENC: Bull. soc. chim., (4) 17, 14 (1915). 

2 Compt. rend., 158, 668 (1914). 

3 Cf. Moors: Chem. News, 71, 82 (1895). 

4 MUuuer: Pogg. Ann., 136, 59 (1869); GuasmR: Z. anorg. Chem., 36, 18 
(1903); TscHucarv and IcHLoPINE: Compt. rend., 159, 62 (1914); BercErR: 
Compt. rend., 158, 1798 (1914); 174, 1341 (1922); cf., however, WOHLER 
and Bauz: Z. Elektrochem., 27, 406 (1921); Levi and TaccuHin1: Gazz. chim. 
ital., 55, 28 (1925). 

’ TowreR and Cooke: J. Phys. Chem., 26, 728 (1922); Towsmr: Ibid., 28, 
176 (1924). 

6 PaaL and Brunuems: Ber., 47, 2200 (1914). 


COPPER, COBALT, NICKEL, SILVER, AND GOLD 155 


Hyprovus NIcKELIC Ox1IpE AND NIcKEL PEROXIDE 


By passing chlorine or bromine through a suspension of nickel 
hydroxide or by warming a nickel salt with an alkali hypochlorite 
or hypobromite, a black precipitate is thrown down which was 
assigned the formula, Ni2O;:3H,0.! This is in error in two 
respects: Not only is the water content indefinite,? but the degree 
of oxidation of the nickel varies with the nature of the oxidizing 
agent, the rapidity of oxidation, and the temperature.* Under 
no conditions is pure hydrous Ni2O; precipitated; and with 
bromine at 0°, the ratio of nickel to oxygen approaches 1:2 
Howell* showed that both hydrous NiO; and NiO2 are formed 
simultaneously during the action of alkali and hypochlorite on 
Ni(OH)e. Since NieOsz is not oxidized, there is a limit to the 
oxygen content of the precipitate. Moreover, unlike cobalt 
dioxide, NiOz is instable, decomposing to NiO without the inter- 
mediate formation of Ni,O3. The rate of decomposition of the 
dioxide is accelerated by heat; but excess alkali stabilizes it by 
adsorption. NiO; has the structure represented by NiO -NiOsz.°® 

A greenish-gray compound having the composition NiO2:xH2O 
is obtained by mixing 30 per cent hydrogen peroxide with a dilute 
alcoholic solution of nickel chloride cooled to 50°, followed by 
the addition of alcoholic potassium hydroxide.* Unlike the 
black dioxide, the green compound behaves like hydrogen 
peroxide. The latter is, therefore, regarded as a true peroxide 


O ye 
Ni¢ | ; and the former as Ni 7 A peroxide of nickel is 
O No 


formed by the electrolytic oxidation of the metal and plays a 
part in the Edison battery.® 


1WacutTerR: J. prakt. Chem., 30, 327 (1843); Veit: Compt. rend., 180, 
21111925). 

2 CARNELLEY and WALKER: J. Chem. Soc., 58, 91 (1888). 

3 BeLLuccr and Ciavari: Atti accad. Lincei, 14, II, 234 (1905). 

4 J. Chem. Soc., 123, 669, 1772 (1923). 

6’ CLark, Asspury, and Wick: J. Am. Chem. Soc., 47, 2661 (1925). 

6 PELLINI and MENEGHINI: Z. anorg. Chem., 60, 178 (1908). 

7 TuBANDT and RIEDEL: Ber., 44, 2565 (1911); Z. anorg. Chem., 72, 219 
(1911); cf., however, TanaTar: Ber., 42, 1516 (1909). 

8 Forrster: Z. Elektrochem., 18, 414 (1907); 14, 17 (1908); RimsENFELD: 
Ibid., 12, 621 (1906); cf., however, ZepNER: [bid., 11, 809 (1905); 12, 463 
(1906); 18, 752 (1907). 


156 THE HYDROUS OXIDES 


The black hydrous dioxide of nickel is peptized by small 
amounts of organic acids, such as acetic, citric, and tartaric, 
forming a very stable colloid. Peptization results simply on 
washing the hydrous oxide with cold water, but the sol obtained 
in this way is not stable.! 


HybDROUS SILVER OXIDE 


By mixing a dilute solution of silver nitrate and KOH in 90 
per cent alcohol at —45°, hydrous silver oxide comes down as a 
flocculent mass almost pure white in color.? As the temperature 
rises, it changes in color from pale brown to brown, owing to 
loss of adsorbed water and agglomeration of the particles. The 
hydrous oxide precipitated at room temperature is brown, but 
becomes black on drying at temperatures as low as 50 to 60°. 
Pure AgeO decomposes slightly even at 100° and it does not give 
up all its adsorbed water until a temperature of 280° is reached; 
accordingly, pure AgeO cannot be obtained.4 

A silver oxide sol is formed both by heating silver wire to red- 
ness and plunging it suddenly into water,’ and by mixing a dilute 
N/40 solution of AgNO; with a slight excess of KOH of similar 
concentration.® <A stable sol is obtained only when the hydrous 
oxide is formed in the presence of a protective colloid such as 
tannin’ or Paal’s sodium protalbinate and lysalbinate.$ 


Tur Hyprous Oxipgs or GOoLpd 


Auric Oxide.—The hydrous oxide formed by treating auric 
chloride with alkali or by decomposing potassium aurate with 
acid is always contaminated with adsorbed alkali or salt which 


1 TuBANDT and RIEDEL: Z. anorg. Chem., 72, 219 (1911). 

2 Bruce: Chem. News, 50, 208 (1884). 

3 Rose: Pogg. Ann., 85, 314 (1852). 

4 MapsEn: Z. anorg. Chem., 79, 200 (1913); Harpin: J. Am. Chem. Soc., 
18, 994 (1898); Lua: Am. J. Sci., (3) 44, 240 (1892). 

5 Kimura: Mem. Coll. Sci., Kyoto Imp. Univ., 5, 211 (1918). 

6 LoTTeRMOSER: J. prakt. Chem., (2) 72, 39 (1905). 

7 SENsBuRG: German Patent 208189 (1907). 

§ Paat and Voss: Ber., 37, 3862 (1904); LorrerMosER: J. prakt. Chem., 
(2) 71, 296 (1905). 


COPPER, COBALT, NICKEL, SILVER, AND GOLD 157 


cannot be removed by washing.! It may be obtained pure by 
heating a solution of gold chloride with magnesia and decompos- 
ing the residue with nitric acid.2, The hydrous oxide is yellow or 
olive green, depending on the method of formation, and becomes 
brown to black on drying. A sample precipitated from potas- 
sium aurate with acid contained more than 8H.O to 1Au.03;3 
and one thrown down from the chloride solution with barium 
hydroxide approached the composition Au,O3-3H20O* when dried 
in vacuum over calcium chloride, and Au2O3;:H2O* when dried 
over phosphorus pentoxide. These data offer no proof of the 
existence of a definite hydrate, and it is altogether likely that - 
none is formed. Like hydrous silver oxide, the gold compound 
decomposes below the temperature at which all of the adsorbed 
water can be driven off. At 160° a composition corresponding 
to the formula, Au2Oxs, gold dioxide,* has been obtained, but the 
identity of such a compound has not been established. 

Aurous Oxide.—lIf a solution of an aurous salt’ is treated with 
potassium hydroxide, a dark-violet precipitate results, which is 
said to be aurous hydroxide; but there is no evidence to support 
this view, and it is probably Au,O with adsorbed water. A 
similar product is obtained by hydrolysis of an aurous salt® or 
by reduction of an auric salt with mercurous nitrate.? The 
hydrous oxide appears to give up all its adsorbed water at 200°, 
and oxygen is not evolved until a somewhat higher temperature. 
The freshly precipitated and washed gel is peptized by shaking 
with water!® forming a fairly stable indigo-blue sol with a brown 
fluorescence.'! The sol shows a maximum adsorption between 


1 ScHOTTLANDER: Liebig’s Ann. Chem., 217, 312 (1883). 

2 PELLETIER: Ann. chim. phys., (2) 15, 113 (1820); Lenumr: J. Am. Chem. 
Soc., 25, 1137 (1903); Morris: Jbid., 40, 917 (1918). 

3 Fiauimr: Ann. chim. phys., (3) 11, 336 (1844). 

4 WitrstEIN: Pharm. Vierieljahr, 15, 21 (1866). 

5 Kriss: Ber., 19, 2541 (1886). 

6 Kriss: Loc. cit.; DuptEy: Am. Chem. J., 28, 61 (1902). 

7 Berzevius: Jahresber., 199 (1846); Kriss: Liebig’s Ann. Chem., 237, 
274 (1887); Prat: Compt. rend., 70, 840 (1870). 

8 ScHOTTLANDER: Liebig’s Ann. Chem., 217, 312 (1883). 

9Fiauier: Ann. chim. phys., (2) 41, 167 (1829); (3) 11, 336 (1844). 

10 Kriss: Ber., 19, 2541 (1886); Liebig’s Ann. Chem., 237, 274 (1887). 

11 Vanino;: Ber., 38, 462 (1905), 


158 THE HYDROUS OXIDES 


y = 586.5 and 597.5; while the maximum adsorption for col- 
loidal gold is y = 535. By boiling the sol or by allowing it to 
stand several days, the dark-violet hydrous oxide is precipitated. 
The stability of the sol would probably be increased by more 
thorough washing, preferably with a centrifuge, before shaking 
with water. 


1 Voce: “Die prakt. Spektralanalyse iridischer Stoffe,” Berlin, 1, 489 
(1889). 


CHAPTER VI 


THE HYDROUS OXIDES OF BERYLLIUM, MAGNESIUM, 
ZINC, CADMIUM, AND MERCURY 


Hyprous BERYLLIUM OXIDE 


Hydrous beryllium oxide is obtained in a gelatinous condition 
by adding ammonia to a solution of beryllium salt. In this 
form it possesses a high adsorption capacity and cannot be 
washed free from the mother liquor. The washing must be 
carried out in the absence of carbon dioxide, since Parsons and 
Roberts! found that the freshly formed oxide will take up as 
much as one-third of an equivalent of the gas. The hydrous 
oxide is readily soluble in dilute acids, alkalies, ammonium 
carbonate, and alkali bicarbonates as well as in solutions of 
beryllium salts. The great solubility of the hydrous oxide in 
sodium bicarbonate serves to distinguish beryllium and to sepa- 
rate it quantitatively from iron and aluminum.” Like most 
gelatinous oxides, the adsorbability and solubility of hydrous 
beryllia decrease slowly on standing at room temperature and 
rapidly at higher temperatures, particularly if heated in a cur- 
rent of steam or in the presence of a solution of ammonia or of 
alkali hydroxide or carbonete.? The ageing is accompanied 
not only by a marked diminution in specific surface but by a 
change from amorphous to a definite crystalline form, as shown 
by x-radiograms.* Béhm and Niclassen® obtained a complete 
series of photographs showing the transformation from the 
amorphous to the stable crystalline modification. In this 


1 Science, 24, 39 (1906). 

2 Parsons: “The Chemistry of Beryllium,” 9, 27 (1908). 

3 HABER and vAN OorpT: Z. anorg. Chem., 38, 377 (1904); vAN OorpT: 
German Patent 165488 (1903). 

4 FREUNDLICH: ‘“ Kapillarchemie,’”’ 2d ed., 456 (1922). 

5Z. anorg. Chem., 132, 1 (1924). 
159 


160 THE HYDROUS OXIDES 


instance the ageing is a concomitant of crystallization as well as 
of the formation of a hydrated oxide or hydroxide from a hydrous 
oxide. A similar thing happens with hydrous aluminum oxide 
and with hydrous cupric oxide under certain conditions; but 
these cases constitute the exceptions rather than the rule. 
Indeed, the hydrous oxides of chromium and zirconium exhibit 
the ageing phenomenon to a marked degree without even under- 
going crystallization. 

The water content of the amorphous gelatinous oxide is indefi- 
nite, depending on the temperature and vapor pressure of the 
surrounding air.t The various hydrated beryllium hydroxides 
described by Atterberg? and others merely represent different 
stages in the removal of adsorbed water from a hydrous oxide. 
When heated a little above 150°, the amorphous compound goes 
over into a different chemical individual, crystalline Be(OH), 
or BeO-H,O, which does not dissociate until a temperature of 
approximately 215° is reached. The crystalline hydroxide 
thrown down by heating a solution of gelatinous oxide in alkali 
has the composition BeO-H.O when dried over sulfuric acid 
at 15° and maintains it up to above 200°. The monohydrate is 
slightly hygroscopic, adsorbing as much as 0.5H.2O from satu- 
rated air but giving it up again in dry air; after losing 0.5H.O 
by heating above 200°, the constitution of the compound is 
definitely changed, for it will then adsorb as much as a mol of 
water in moist air, giving it up entirely in dry air; similarly, 
by heating to 280°, the water content is reduced to 0.13H.O 
and again it will adsorb a mol or more of water, which it gives up 
in dry air until the composition is BeO-0.18H.0. X-radiograms 
of the monohydrate and of the compound heated to 280° should 
be obtained to determine whether the increased power of the 
latter to adsorb water is due to the transformation to an amor- 
phous compound or one with a different crystal lattice. When 
either the gelatinous oxide or the crystalline hydroxide is heated 
to red heat, the anhydrous oxide no longer adsorbs water, 
probably owing to the cutting down of the extent of surface by 
sintering. 

1Van BEMMELEN: J. prakt. Chem., (2) 26, 227 (1882); Z. anorg. Chem., 
18; 126 7189S 

2 Kéngl Svenska Vet. Akad, Hand., 12, 1 (1878). 


BERYLLIUM, MAGNESIUM, ZINC, CADMIUM, MERCURY 161 


The solubility of the gelatinous oxide in dilute alkalies and 
alkali bicarbonates has been determined by a number of investi- 
gators, but the values are not constant, since they depend on the 
method of preparation and the age of the sample, as recognized 
clearly by Haber and van Oordt.? Bleyer and Kaufmann? 
recognize three modifications of the oxide; A, the readily soluble 
gelatinous form to which they assign the formula, 2BeO-H,O or 
H-Be20;; B, the less soluble crystalline monohydrate thrown 
down from alkali solutions; and C, a still less soluble monoydrate 
obtained by drying C. Obviously, one may obtain intermediate 
modifications with intermediate properties between A and B and 
between B and C, if one desires. In other words, the gelatinous 
precipitate may show a continuous variation in properties from 
the instable, voluminous, and highly soluble oxide to the stable, 
relatively insoluble, and slightly adsorptive crystalline mono- 
hydrate. Similarly the crystalline compound possesses prop- 
erties which may vary continuously through certain limits 
depending on the size of the crystals and the physical structure 
of the mass. The rapidity with which the highly adsorptive 
gelatinous oxide goes over into a slightly adsorptive condition, 
particularly at 100°, may account for Prud’homme’s* observation 
that beryllia does not act as a mordant. 

Beryllium hydroxide possesses a very slight acidic character® 
and forms beryllates with alkalies. The salts, Be(OK)2 and 
Be(ONa)2, have been obtained in a crystalline form from alco- 
holic solution. About 40 per cent of an aqueous solution of the 
sodium salt is hydrolyzed in a 0.1 N solution.’ According 
to Hantzsch the solution of hydrous beryllia in alkali is partly 
beryllate. From a concentrated solution, Be(OH): precipitates 
spontaneously on standing for a long time or rapidly on heating. 


1GmeELIN: Pogg. Ann., 50, 175 (1840); ScHarraotscu: Jbid., 50, 183 
(1840); Hantzscu: Z. anorg. Chem., 30, 289 (1902); RupEnBAvER: [bid., 30, 
331 (1902); Woop: J. Chem. Soc., 97, 878 (1910). 

2Z. anorg. Chem., 38, 377 (1904). 

3Z. anorg. Chem., 82, 71 (1918). 

4 Bull. soc. chim., [8] 18, 509 (1895). 

6 Hantzscu: Z. anorg. Chem., 30, 289 (1902); Ley: Z. physik. Chem., 30, 
218 (1899). 

6 Kriss and Moraur: Liebig’s Ann. Chem., 260, 173 (1890). 

7™Woopn: J. Chem. Soc., 97, 878 (1910). 


162 THE HYDROUS OXIDES 


It seems likely that the first step in the solution process is pep- 
tization by preferential adsorption of hydroxyl ion. This is 
followed by the formation of beryllate, the breaking down of 
which gives granular crystals of the difficullty soluble crystalline 
hydroxide. As Ostwald! points out, the stable hydroxide 
is not present in the original solution which soon becomes 
supersaturated with respect to it; but precipitation can commence 
only after the first traces have come down, a step that occurs 
slowly at ordinary temperatures, but rapidly when the solution 
is heated. Unlike alumina, hydrous beryllia is neither peptized 
nor dissolved by ammonia? or by methyl- or ethylamine.* 

To prepare beryllium hydroxide free from adsorbed material, 
Parsons, Robinson, and Fuller* dissolve the impure gelatinous 
hydrous oxide in ammonium carbonate, and precipitate the basic 
carbonate with steam. ‘The latter compound is decomposed by 
boiling with frequently renewed portions of water while a stream 
of air is passed through the liquid. ‘The product is almost free 
from adsorbed ammonia’ or carbonate. 

Concentrated solutions of normal beryllium salts can dissolve 
2 to 6 equivalents of hydrous beryllia; thus, the oxalate or sulfate 
dissolves nearly 3 equivalents, the chloride 4, and the acetate 
nearly 6. In every case the hydrous oxide precipitates on dilu- 
tion, although the precipitation is not complete. This solution 
is not due to the formation of a molecular complex nor is there 
any evidence of sol formation.® Parsons’ believes the dissolved 
beryllium salt merely acts as a solvent for hydrous beryllium 
oxide in the same manner as a water solution of acetic acid dis- 
solves camphor which is itself insoluble in water; or as a solution 
of potassium iodide dissolves iodine without forming the hypo- 
thetical KIs. 

If hydrous beryllium oxide is precipitated in the presence of 
boric acid, the distribution of the latter between the hydrous 

1 “The Principles of Inorganic Chemistry,’’ London, 546 (1902). 

2 WEEREN: Pogg. Ann., 92, 91 (1854). 

3 Renz: Ber., 36, 2751 (1903). 

4Cf. Brirron: J. Chem. Soc., 121, 2612 (1922). 

5 J. Phys. Chem., 11, 651 (1907); cf. Parsons and Barnzs: J. Am. Chem. 
Soc., 28, 1589 (1906). 

6 PARSONS, RoBINSON and Futumr: J. Phys. Chem., 11, 651 (1907). 

7 J. Phys. Chem., 11, 659 (1907). 


BERYLLIUM, MAGNESIUM, ZINC, CADMIUM, MERCURY 163 


oxide and water is independent of the concentration of boric acid, 
both at 20 and 100°. Similarly, the composition of the precipi- 
tate formed by mixing sodium borate and beryllium sulfate 
varies with the concentration and the relative proportion of the 
reacting substances. It is evident, therefore, that the so-called 
beryllium borates,‘ like a large number of alleged beryllium com- 
pounds,? are, in reality, solid solutions of boric acid and hydrous 
beryllium oxide.* Arsenious oxide likewise seems to form solid 
solutions with beryllium oxide at 100°; but at room temperature, 
the acid is adsorbed by the gel giving a well-defined adsorption 
isotherm.* A freshly formed hydrous oxide adsorbs’ acid dyes, 
such as eosin and Congo red, the latter being taken up more 
strongly than by hydrous alumina. It also adsorbs invertin 
and amylase more strongly than alumina. Basic dyes, such as 
methylene blue, are adsorbed very slightly, and the same is 
true for acetic acid, grape sugar, and tributyrin. The adsorption 
capacity of gels decreases rapidly with age on account of the 
rapid change from a gelatinous to a granular structure. 

While colloidal solutions of hydrous beryllium oxide have not 
been described in detail, B6hm and Niclassen® prepared a clear 
concentrated sol by peptizing freshly made gelatinous oxide with 
a small amount of 0.05 N hydrochloric acid. Since the gelati- 
nous oxide runs through the filter paper when an attempt is made 
to wash out adsorbed salts, there is little doubt that a pure 
sol could be prepared by thorough washing of the hydrous gel 
using the centrifuge or supercentrifuge. 

So far as known, hydrous beryllium oxide has no industrial 
applications, but anhydrous beryllia finds an important applica- 
tion in the manufacture of incandescent gas mantles. A very 
small amount of BeO gives greater strength to the mantle and 
so is of particular value for mantles which are given special shapes, 
such as those for use with a pressure system. In this connection 
it may be mentioned that beryllium nitrate is sometimes added 

1 BLEYER and Pazuski: Kolloid-Z., 14, 295 (1914). 

2 Parsons: ‘‘The Chemistry of Beryllium,” 69 (1908). 

3 Kriss and Morant: Ber., 23, 727 (1890). 

4 BueYerR and Miuer: Arch. Pharm., 251, 304 (1913); Z. anorg. Chem., 
75, 285 (1918). 

> KLEEBERG: Kolloid-Z., 87, 17 (1925). 

6 Z. anorg. Chem., 132, 5 (1924). 


164 THE HYDROUS OXIDES 


to the collodion for mantle coating to increase the protection 
given the mantle.! 

The crystals of beryllia obtained from an electric are furnace 
are almost as hard as corundum, and so it is sometimes mixed 
with other substances as an abrasive.! The oxide would also 
seem to possess certain advantages over magnesia as a refractory 
for crucibles. It has a high melting point, 2450°, and after cal- 
cination it resists acid corrosion much more effectively than 
magnesia.2 The oxide has also shown some promise as a body 
in paints and in the manufacture of certain dental products and 
synthetic gems. 


Hyprous Maanesium HypRoxiIpE 


Magnesium hydroxide in a flocculent. hydrous condition is 
formed by the action of water on magnesia obtained from the 
naturally occurring carbonate, magnesia alba.* The precipi- 
tated mass is not a hydrous oxide as is so frequently the case, 
but is a hydrous hydrate* made up of very finely divided particles 
which adsorb alkali so strongly® that its presence prevents the 
adsorption of sulfate and chloride. The oxide is more soluble 
when first formed, going over to a less soluble crystalline form 
quickly when the magnesium ion concentration is high, and more 
slowly when it is low.’ -‘X-radiograms show the microcrystalline 
particles to possess a structure identical with natural brucite.*® 
Large crystals of the hydrate are formed by heating magnesium 
chloride with an excess of potash in a limited volume of water.?® 

The flocculent hydroxide dried over sulfuric acid at 100 to 
200° adsorbs more than 1.5 H20, which it gives up in a dry atmos- 


1Cf, James: Metal Ind., 11, 66 (1917). 

2 BERZELIUS: Sioeigere J., 15, 236 (1815); Leprav: ‘Com rend., 123, 
818 (1896); Ann. chim. phys., (7) 16, 457 (1899). 

3 DEviLLE: Compt. rend., 61, 975 (1865); Drrrn: Ibid., 73, 191 (1871). 

4VaN BEMMELEN: J. prakt. Chem., 26, 238 (1882). 

5 GROUVELLE: Ann. chim. phys., (2) 17, 354 (1821); MarcHuanp and 
ScutiRer: J. prakt. Chem., (1) 50, 385 (1850). 

6 Parren: J. Am. Chem. Soc., 25, 186 (1903). 

7 GJALDBAEK: Z. anorg. Chem., 144, 145, 269 (1925). 

8 BOum and NicuassEen: Z. anorg. Chem., 132, 6 (1924). 

9 Dre ScHULTEN: Compt, rend., 101, 72 (1885), 


BERYLLIUM, MAGNESIUM, ZINC, CADMIUM, MERCURY 165 


phere.t The amount of adsorbed water taken up decreases with 
the temperature of ignition and the anhydrous oxide obtained 
at high temperatures hydrates very slowly. Campbell? burned 
magnesite between 600 and 800°, obtaining an impure oxide which 
hydrates completely in 3 days. Between 1000 and 1100°, the 
magnesia was said to undergo a change resulting in a marked 
decrease in the rate of hydration until at 1450°, about the tem- 
perature used in burning Portland cement, the oxide, immersed 
in water for 18 months, combines with 60 per cent of that neces- 
sary to form MgO-H.O. Le Chatelier* gave 1600° as the trans- 
formation temperature, and Parravano and Mazzetti* placed it 
at 800°, at the same time calling attention to the effect of impur- 
ities on the transformation temperatures; thus, ferric oxide 
hastens it. Mellor® pointed out the absence of a definite 
transformation temperature and showed the change to proceed 
more quickly the higher the temperature of calcination. Mellor 
attributed the change to a conversion from amorphous to crystal- 
line periclase; but this cannot be the case, as Hedvall® found the 
oxide formed at various temperatures to have a _ cubic- 
lattic crystal structure which underwent no change on heating. 
The specific gravity of calcined magnesia varies, however, between 
3.0 and 3.6, depending not only on the method of preparation 
but on the temperature of calcination. The low-temperature — 
low-specific-gravity oxide not only reacts much more rapidly 
with water than the oxide formed at high temperatures, but the 
former possesses a greater adsorption capacity for gases and 
moisture, and dissolves more rapidly in acids.’ Although the 
melting point of magnesia is in the neighborhood of 2500°, it 
undoubtedly sinters at a much lower temperature, and this change 
in physical character probably accounts for the difference in 
reactivity of the oxide ignited at different temperatures. 


1 Van BemMME EN: “ Die Absorption,’’ 369 (1910). 

2 J. Ind. Eng. Chem., 1, 665 (1909). 

3 Compt. rend., 102, 1248 (1883). 

4 Atti accad. Lincei, (5) 30 I, 63 (1921). 

5 Trans. Ceram. Soc., 16, 85 (1917). 

6 Z. anorg. Chem., 120, 327 (1922). 

7 Drrre: Compt. rend., 78, 111, 191, 220 (1871); ANpERson: J. Chem. Soc., 
87, 257 (1905). 


166 THE HYDROUS OXIDES 


Magnesia Cement.—It is an interesting fact that magnesia 
prepared by heating the chloride or nitrate to redness possesses 
hydraulic properties similar to Portland cement in that it sets 
to a rigid mass when mixed with a limited quantity of water.! 
If the nitrate is calcined at as low a temperature as 350°, the 
resulting magnesia will not set; if calcined at 440 to 500°. 
the magnesia hardens under water and at the end of 2 months is 
like polished marble; but if heated to 1200° or more, the oxide 
loses its power to set. The oxide obtained by gentle ignition of 
natural magnesite also possesses hydraulic properties but that 
obtained from synthetic carbonates will not set, although it 
appears to react readily with water. This difference cannot be 
due to the presence of impurities in the natural product, since 
an hydraulic oxide is formed by converting the synthetic car- 
bonate to nitrate and igniting the latter. As willbe discussed in 
Chap. XVIII, the setting of such substances as Portland cement 
and plaster of Paris involves the formation of a gel structure,’ 
and the same is probably true in the setting of magnesia. The 
temperature of ignition and the structure of the calcined sub- 
stance determine the physical character of the oxide, and these, 
in turn, determine the rate of hydration and the nature of the 
resulting product. As in the preparation of jellies by precipi- 
tation, a suitable rate of agglomeration of highly hydrous par- 
ticles is essential for obtaining a firm jelly structure. 

Magnesia possessing setting properties is sometimes used in 
conjunction with lime for mortar making in districts where only 
magnesium limestone is available. Similarly, gently calcined 
magnesia is mixed with crushed dead-burnt magnesia in manu- 
facturing firebricks so widely used in the basic Bessemer steel 
process. The hydraulic magnesia gives plasticity to the paste 
formed by mixing the materials with water to permit of molding. 

Sorel’s magnesia cement consists of a mixture® of magnesia with 
a concentrated solution of magnesium chloride, sp. gr. 1.16 to 
1.26. This sets in a short time to a con mass made up of 


1 DevitueE: Compt. rend., 61, 975 (1865); ScHWARZ: ingles polytech. J., 
186, 25 (1867); Knapp: Ibid. 202, 513 (1872). 

2 Cf. MICHAELIS: Kollota-Z., 5, 9 (1909); 7, 320 (1910); KriseRMANN: 
Kolloidchem. Bethefte, 1, 423 (1910). 

3 SOREL: Compt. rend., 65, 102 (1867), 


BERYLLIUM, MAGNESIUM, ZINC, CADMIUM, MERCURY 167 


minute interlacing crystals! of what has been assumed to be 
basic chloride. People are unable to agree on the formula of the 
hypothetical salt, and it is probably only an indefinite solid 
solution of magnesium oxide and chloride. The chloride may be 
dissolved out completely with boiling water, leaving hard mag- 
nesium oxide. ‘The cement can be prepared by adding water to a 
suitable mixture of dry components.? It possesses marked 
mechanical strength and is used for cementing glass and metal 
and for making artificial stones; e.g., xylolith is made from saw- 
dust, cement, and water. 

The tendency of calcined magnesia to take up water and 
expand is of importance in the cement industry, since the presence 
of as much as 2 to 3 per cent of uncombined magnesia would give 
a concrete that would disintegrate from excessive expansion.? 

In addition to the applications mentioned, hydrous magnesium 
hydroxide has been substituted for charcoal as a clarifier in the 
refining of sugar.4 Its mild basic action has been utilized in 
pharmaceutical preparations as an antacid. Milk of magnesia 
is a fairly stable suspension of the hydrous oxide that is widely 
employed as a mouth wash; in the preparation of modified milk 
for infants; and in combating hyperacidity of the stomach. 

Rhythmic Bands.—The precipitation of magnesium hydroxide 
in gelatin in the form of rhythmic bands has been investigated 
quantitatively by Popp. When ammonia diffuses into gelatin 
containing magnesium chloride, it is found that with increasing 
concentration of magnesium salt, the rings increase in number 
and thickness, and the space between them decreases; with dimin- 
ishing ammonia concentration, the rings decrease in number and 
thickness, and the space between them increases; adding ammo- 
nium chloride causes the number and thickness of the rings to 
decrease and the space between them to increase; with diminishing 
gelatin concentration, both the rings and the space between them 
increase, the number remaining the same. The rhythmic precip- 

1 LUHMANN: Chem. Ztg., 25, 345 (1901); Krinaer: [bid., 34, 246 (1910). 

2 Cf. KRANER: German Patent 143933 (1902); Lyre and Tarrers: Brit- 
ish Patent 11545 (1890). 

3 CAMPBELL and WuitTe: J. Am. Chem. Soc., 28, 1273 (1906); CAMPBELL: 
J. Ind. Eng. Chem., 1, 665 (1909). 


4 Hake: J. Soc. Chem. Ind., 2, 149 (1883). 
5 Kolloid-Z., 36, 208 (1925), 


168 THE HYDROUS OXIDES 


itation takes place also in clay, agar, silica gel, fine sand, and 
glass beads in water. To account for these and other Liesegang 
phenomena, Wolfgang Ostwald! postulates the existence of three 
principal diffusion waves in all reacting systems giving typical 
periodic precipitates: The added electrolyte diffuses into the gel; 
the electrolyte in the gel diffuses outward; and the electrolyte 
produced by the reaction may diffuse in both directions. In 
many instances the soluble reaction product possesses a higher 
rate of diffusion than one or both of the reactants. Ostwald 
assumes further that many and probably all reactions giving 
Liesegang rings are balanced reactions. Precipitation, therefore, 
depends on certain critical concentrations of reactants which 
vary over wide ranges through the interference of diffusion waves. 
In support of the theory, it was shown that many Liesegang 
rings are destroyed by subsequent introduction, by diffusion, of 
the electrolyte produced in the reaction. ‘Thus, bands of mag- 
nesium hydroxide are destroyed by allowing ammonium chloride 
to diffuse into the gel supporting them. The converse of rhyth- 
mic precipitation, namely rhythmic solution, may sometimes 
be produced by adding a reaction product. Thus a uniform 
precipitate of lead sulfate in gelatin gel containing ammonia 
is converted into rings by the interdiffusion of concentrated 
ammonium chloride. Continuous precipitation results if one 
reactant is replaced by a compound not giving a balanced 
reaction, as evidenced by the failure to get bands when alkali 
is substituted for ammonia in the precipitation of magnesium 
hydroxide in gelatin. The distribution of chloride ions in a 
gelatin jelly containing magnesium chloride was found after 
the diffusion of ammonia, to show periodic variation between 
values much higher and much lower than those in the original gel. 

Wolfgang Ostwald’s theory of rhythmic banding is merely 
an extension of Holmes’? diffusion theory based on Frick’s law 
of diffusion. The influence of such phenomena as supersatura- 
tion,*® peptization and coagulation of the precipitate,* adsorption 


1 Kolloid-Z. (Zsigmondy Festschrift), 36, 380 (1925). 

2 J. Am. Chem. Soc., 40, 1187 (1918). 

3 OstwaLD: ‘‘Lehrbuch allgem. Chemie,”’ 2d ed., 2, 778. 

4 FREUNDLICH: ‘‘ Kapillarchemie,”’ 2d ed., 1009 (1922); Sen and Duar: 
Kolloid-Z., 34, 270 (1924). . 


BERYLLIUM, MAGNESIUM, ZINC, CADMIUM, MERCURY 169 


of reacting solutes by the precipitate,' etc. is looked upon as a 
secondary factor in the banding process. 

Stable sols of hydrous magnesium hydroxide in water have not 
been prepared without the aid of a protective colloid; but a 
typical sol of great stability is formed by shaking magnesia with 
methyl! alcohol. 


Hyprovus ZINc OXIDE 


The voluminous precipitate obtained by adding the calculated 
amount of ammonia or alkali to a solution of zinc salt is hydrous 
zinc oxide, the amount of adsorbed water depending on the 
exact method of formation, the temperature, and the age of the 
sample.* If the precipitation is carried out at 100°, it contains 
less than 1 per cent of adsorbed water. Although the oxide 
newly formed in the cold is a transparent gel,* it quickly becomes 
flocculent and later powdery, the change being accompanied by a 
gradual transformation into the crystalline state. The hydrous 
oxide ages more rapidly if precipitated from chloride rather 
than from nitrate; and in the presence of alkali rather than water. 
As in the case of hydrous beryllium oxide, the microcrystalline 
mass formed on standing in the cold always contains more water 
than corresponds to ZnO-H,O, and in this state, it is probably 
hydrous zinc hydroxide. Kaufmann® observed a gradual loss 
of water on heating a precipitated hydrous zine oxide to 125°, 
where it had the composition Zn(OH). which it maintained to 
180°, and then broke down gradually, giving anhydrous ZnO 
atalowred heat. There are no hydrates of Zn(OH)., as assumed 
by De Forcrand’ and Boedecker.® 


1 BraDForD: Biochem. J., 10, 169 (1905). 

2 NEUBERG and REewa.p: Kolloid-Z., 2, 354 (1908). 

3 GouDRIAAN: Rec. trav. chim., 39, 505 (1920). 

4LinpDER and Picton: J. Chem. Soc., 61, 130 (1892). 

-5 Fricke and AHRNDTs: Z. anorg. Chem., 134, 344 (1924); Frickn: Jbid., 

136, 48 (1924); BGum and Nicuassen: [bid., 182, 1 (1924). 

6 Dissertation, Miinchen, 69 (1913); cf. Pascau: Compt. rend., 177, 765 
(1923). 

7 Compt. rend., 135, 36 (1902). 

8 Liebig’s Ann, Chem., 94, 358 (1855), 


170 THE HYDROUS OXIDES 


The hydrous precipitates adsorb chloride, nitrate, and espe- 
cially sulfate! so strongly that they cannot be purified completely 
by washing.? Large crystals of Zn(OH)s, exhibiting a very slight 
adsorption capacity, precipitate spontaneously from the alkali 
solution prepared in a variety of ways. ‘Thus, Goudriaan 
obtained long prismatic needles from a solution of normal zinc | 
sulfate to which normal potassium hydroxide was added until 
the precipitate first formed just failed to redissolve; and Fricke 
and Ahrndts obtained the usual dense rhombic crystals by 
diluting a solution of the hydroxide in strong alkali. 

The newly formed gel dissolves readily in alkali, 1 atom of 
Zn" being taken up by approximately 6 of OH’.* On account of 
the ageing of the oxide, the solubility in alkali and ammonia is 
less the older and less hydrous the preparation. ‘The variation 
in the solubility has naturally led to the assumption that the 
oxide exists in different polymerized forms or allotropic modifica- 
tions. Klein® recognizes an easily soluble form 2ZnO- H,O0 and 
two insoluble forms having the composition, Zn(OH):, analogous 
to Bleyer and Kaufmann’s A, B, and C beryllium oxides. But as 
in the latter case, the solubility is not definite but varies con- 
tinuously from the loose highly gelatinous to the most massive 
granular form. 

Although the alkali solution of hydrous Zn(OH). has been 
the subject of repeated investigations during the past 25 years, 
there is still a difference of opinion as to the exact nature of such 
solutions. On account of the very weak acidic character of 
Zn(OH)e, Hantzsch® believed that alkalies peptize the latter, 
forming an insoluble sol from which most of the hydroxide 
precipitates on standing, leaving the remainder in solution as 


1 Kuritorr: Chem. Zentr., 1222 (1901). 

2 GoUDRIAAN: Rec. trav. chim., 39, 505 (1920); Frickm and AHRNDTs: 
Z. anorg. Chem., 134, 344 (1924); Lorenz: Jbid., 12, 489 (1896); Hatt: 
Am. Chem. J., 19, 901 (1897). 

3’ RUBENBAUER: Z. anorg. Chem., 30, 331 (1902); Hmrz: Ibid., 28, 274 
(1901). 

4 Hantzscu: Z. anorg. Chem., 30, 289 (1902); Kunscuert: [bid., 41, 337 
(1904). . 

5Z. anorg. Chem., 74, 157 (1912); cf. De Forcranp: Compt. rend., 134, 
1426 (1902); 135, 36 (1902); Masson: Bull. soc. chim., [3] 15, 1104 (1896). 

6 Z. anorg. Chem., 30, 300 (1902). 


BERYLLIUM, MAGNESIUM, ZINC, CADMIUM, MERCURY 171 


zincate. As Hantzsch worked with dilute alkali solutions, he 
was probably right in concluding that most of the hydroxide 
was peptized; but diffusion experiments! and electrometric 
measurements” on solutions in concentrated alkali showed the 
presence of alkali zincate. The more concentrated the alkali, 
the more hydroxide it will take up and the more zincate will 
form.* Goudriaan* determined the 30° isotherm for the system, 
Naz2O-ZnO-H,0. The saturation concentration increases 
rapidly to the triple point, ZnO-Na2ZnO,-4H.O, where the com- 
position of the solution in weight per cent is 27.8 per cent Na2O 
and 16.5 per cent ZnO. The zincate forms well-developed 
crystals decomposed by water and is stable from the triple point 
to the quadruple point, NaZnO,:4H.,O-Na,O-3H.0-H.0, at 
39.2 per cent Na2O and 9.7 per cent ZnO. Sodium zincate forms 
an incongruent solution, the addition of water to the solid salt 
or the dilution of the solution causing ZnO to precipitate. This 
accounts for a number of so-called sodium zincates®> which are 
either metastable or non-existent. While Na,ZnOz appears to be 
the stable salt in strong alkali solution, electromotive determin- 
ations of fixed H'ion, on adding sodium hydroxide to solutions 
of zine salts, indicate the formation of acid zincate in rela- 
tively dilute alkali. Such solutions always contain colloidal 
zine hydroxide stabilized by preferential adsorption of hydroxyl 
ion. Fricke and Ahrndts claim that potassium hydroxide forms 
chiefly KHZnO, even in concentrations above 8 N. 

By dipping red-hot zine into water, a sol is formed consisting 
of both colloidal zinc and zine hydroxide.’ A dilute sol results 
by allowing zinc to stand in water for a long time in contact with 


1 CorrrELu: Z. physik. Chem., 42, 418 (1902); Kaurmann: Dissertation, 
Minchen, 45 (1913); KremMann: Z. anorg. Chem., 35, 48 (1903). 

2Duroir and Groset: J. Chim. phys., 19, 324 (1921); Fricke and 
AuRNDTS: Z. anorg. Chem., 134, 344 (1924). 

3 KEIN: Z. anorg. Chem., 74, 157 (1912); Rupensaver: Jbid., 30, 331 
(1902); Woop: J. Chem. Soc., 97, 878 (1910). 

4 Rec. trav. chim., 39, 505 (1920). 

5 H.g., see Comey and Jackson: Am. Chem. J., 11, 145 (1889). 

6 HILDEBRAND and Bowers: J. Am. Chem. Soc., 38, 785 (1916); cf. also 
Kunscuert: Z. anorg. Chem., 41, 337 (1904); Forrster: Z. Elektrochem., 
6, 301 (1899). 

7 Kimura: Mem, Coll. Sci., Kyoto Imp. Univ., 6, 211 (1913). 


172 THE HYDROUS OXIDES 


air.! With the exception of alkali-peptized colloids, concentrated 
sols have been obtained only in the presence of protective col- 
loids, such as potassium soaps? and sodium _ protalbinate.* 

Zine oxide in the finely divided or colloidal state finds its most 
important application in the anhydrous rather than the hydrous 
condition. Thus, zine white alone, or mixed with finely ground 
silica or calcium carbonate and ground with linseed oil, forms a 
white paint that does not discolor in the presence of H2S. A 
suitable mixture of zinc white and of finely divided zine hydrox- 
ide precipitated in the cold is said to form a useful enamel 
pigment.4 Zine oxide has a mild antiseptic action, and a sol 
consisting of the oxide, gutta percha, and Venice turpentine 
is applied to cloth in the manufacture of surgeons’ adhesive 
tape. Like magnesia, a wet mixture of zine oxide and chloride 
sets to a solid gel. A strong dental cement consists of a mixture 
of zine oxide and aluminum phosphate. The oxide also finds 
some applications in face powders, in glazes, and as a filler in 
oilcloth and celluloid; but by far the greatest demand is as a 
filler and pigment in rubber goods, especially automobile tires. 


Hyprous CADMIUM OXIDE 


Hydrous cadmium oxide precipitates in a very voluminous 
and highly hydrous form when a concentrated solution of 
cadmium salt is treated with alkali. The precipitate loses 
water on heating, becoming a flocculent microcrystalline mass 
of hydrous Cd(OH)»2. The purest form is obtained from nitrate 
solution, since it adsorbs nitrate less strongly than chloride or 
sulfate. Like the corresponding zine compound, cadmium 
hydroxide is soluble in excess ammonia; but unlike the former, it 
is only slightly soluble in dilute alkalies. Hot, highly concen- 
trated solutions of potassium hydroxide carry considerable 
amounts into solution from which hexagonal plates of Cd(OH), 

1 TRAUBE-MENGARINI and Scaua: Kolloid-Z., 10, 115 (1912); NorpENSON: 
Kolloidchem. Bethefte, T, 106 (1915). 

2 KurRILoFF: Z. Hlektrochem., 12, 213 (1906); Rora: German Patent 
228139 (1908). 

3 PaaLt and HartMann: Ber., 51, 894 (1918); AmpeRGcER: German Patent 
229306 (1909). 

4 Joannis: J. Soc. Chem. Ind., 25, 486 (1906). 

6 FoLuLENIvs: Z, anal. Chem., 13, 272 (1874). 


BERYLLIUM, MAGNESIUM, ZINC, CADMIUM, MERCURY 173 


erystallize.! | Alkali sulfides react with the voluminous oxide 
formed in the cold, giving yellow cadmium sulfide, and with the 
aged oxide formed in the hot, giving red cadmium sulfide. Since 
the yellow and red sulfides were thought to be polymers, Biichner? 
assumed the existence of two forms of Cd(OH)2; but it now 
appears that the difference in color of the sulfides is not due to 
polymorphism or to crystal structure, but to a difference in the 
size and nature of the surface of the particles. Rapid action of 
the voluminous compound with alkali sulfides gives small 
yellow particles, while slower action with the denser aged 
hydroxide gives larger particles that appear red. 


Hyprovus OxIpEs oF MERCURY 


Mercuric Oxide.—Hydrous mercuric oxide is thrown down as 
a yellow flocculent mass on adding alkali to a cold mercuric 
solution. It does not form the monohydrate or hydroxide 
HgO- H20, as claimed by Carnelley and Walker,‘ nor does it retain 
its adsorbed water very strongly, but is readily dried to the 
anhydrous oxide.® If the yellow oxide is boiled with aqueous 
solutions of salts or the dried oxide is heated, the color changes 
to orange red. This red compound is formed directly by the 
thermal decomposition of mercuric nitrate. As usually obtained, 
the yellow oxide decomposes at a lower temperature, is more 
soluble in water, and reacts more readily with acids, alkalies, 
and salts than the red compound. ‘These distinct differences 
in physical and chemical properties were attributed by Gay 
Lussac® and later by W. Ostwald’ and others to a difference 
in the degree of fineness of the particles, the greater activity of 
the yellow oxide resulting from the greater surface of the smaller 


1 Dr ScHuLTEeN: Compt. rend., 101, 72 (1885). 

2 Ber., 20, 681 (1887); cf. KuosuLKkorr: J. prakt. Chem., (2) 39, 412 
(1887). 

3 ALLEN and CrensHaw: Am. J. Sci., (4) 34, 341 (1912). 

4 J. Chem. Soc., 58, 59 (1888); cf. ScHarrNneR: Liebig’s Ann. Chem., 61, 
182 (1844). 

’Scnocu: Am. Chem. J., 29, 321 (1902); cf. Mitton: Ann. chim. phys., 
(3) 18, 33 (1846). 

6 Compt. rend., 16, 309 (1843). 

7Z. physik. Chem., 18, 159 (1895); 34, 495 (1900); Scuick: /bid., 42, 155 
(1903); Varer; Compt. rend., 120, 622 (1895). 


174 THE HYDROUS OXIDES 


particles. This view was called in question by Glazebrook and 
Skinner! and by Cohen? who showed that the E.M.F. of the 
chain: Hg| HgO red, KOH, HgO yellow|Hg, was 0.685 millivolt,. 
indicating the existence of two isomeric modifications of the 
oxide; but Ostwald and Allmand?® traced these results to the 
variation in solubility of particles of different size. Schoch® 
attributed the difference in properties to a difference in crystal 
structure, the yeilow oxide consisting of quadratic plates and 
the red of prisms. Allmand confirmed Schoch’s observation but 
showed conclusively that either type of crystal may be yellow 
or red, depending altogether on the state of subdivision of the 
particles. 

A stable yellow sol is obtained by precipitating hydrous 
mercuric oxide in the presence of Paal’s® sodium salt, of protalbinic 
and lysalbinic acids which act as protective colloids. After 
dialysis, this is agglomerated by acids and certain salts, giving 
a gelatinous precipitate. By adding mercuric chloride to a 
normal solution of potassium hydroxide containing 40 cubic 
centimeters of acetone, a sol is obtained which sets to a firm jelly 
on standing, the time required depending on the concentration of 
sol.? The setting may be hastened by adding a small amount 
of acid or by heating; but too much heating causes agglomera- 
tion to a gelatinous precipitate. For some unknown reason, the 
presence of even a small amount of mercurous salt seems to retard 
or prevent jelly formation. 

Mercurous Oxide.—Hydrous mercurous oxide, obtained by 
adding alkali to a mercurous salt solution, cannot be obtained 
free from mercuric oxide. Bird’ claims to get mercurous hydrox- 
ide by mixing mercurous nitrate with alcoholic potassium hydrox- 
ide at —42°; but this has not been proved. 


1 Proc. Roy. Soc., 51, 60 (1892). 

2Z. physik. Chem., 34, 69 (1900). 

3 Z. Hlektrochem., 16, 254 (1910). 

4 Huuett: Z. physik. Chem., 37, 385 (1901). 

5 Am. Chem. J., 29, 321 (1902). 

6 Ber., 35, 2219 (1902); cf. KaLtuE and Co.: Z. angew. Chem., 20, 1374 
(1907); May: German Patent 248526 (1911). 

7 Bunce: J. Phys. Chem., 18, 269 (1914); Reynoups: Proc. Roy. Soc., 
19, 431 (1871). 

8 Am. Chem. J., 8, 426 (1886); cf. Rercuarn: Ber., 30, 1914 (1887), 


CHAPTER VII 
THE HYDROUS OXIDES OF SILICON AND GERMANIUM 


Hyprovus SILICON DIOXIDE 
SILICA GEL 


Composition.—The classic investigations of van Bemmelen 
on the composition of the hydrous oxides were climaxed by his 
exhaustive study of the hydration and dehydration of hydrous 
silica! thrown down from alkali silica solutions with dilute hydro- 
chloric acid. A silica jelly containing 300 mols of water to 1 of 
silica is very soft, and when broken into pieces, it flows together 
like a viscous liquid. A gel with a water content of 30 to 40 
mols is brittle; and with 6 mols, it can be pulverized, giving an 
apparently dry powder. On further dehydration, the vapor- 
pressure curve drops continuously, giving no indication of a 
definite hydrate. The highly hydrous oxide is almost perfectly 
clear, but when the water content drops to a point usually 
between 1.5 and 3.0 mols, depending on the method of preparation 
and the history of the sample, the gel becomes opaque and 
chalky but clears up once more when the water content is reduced 
to 0.5 to 1.0 mol. The clouding is due to the appearance of air 
bubbles in the pores of the gel and lasts until the pores are com- 
pletely filled with air. Owing to capillary action, the water which 
evaporates from the outer surface of the capillaries is replaced 
from the inside of the gel leaving a vapor space in the center of 
the jelly and thus producing an opacity which lasts until the 
pores are free from capillary water. The remaining 0.5 to 1.0 
mol is adsorbed very strongly on the surface of the particles and 
can be removed only by heating to a relatively high temperature. 

In Fig. 11 is given van Bemmelen’s schematic representation 
of the pressure-concentration relations at 15° for a freshly formed 
hydrous silica. The A curves represent the first dehydration 

‘Van BEMMELEN: “Die Absorption,” 196, 214, 232 (1910). 

175 


176 THE HYDROUS OXIDES 


over sulfuric acid; and the Z curves are for an oxide which has 
_ been dehydrated once, more or less completely. The direction 
of the arrows shows whether water is being taken up or given 
off. Starting with a fresh gel, the vapor pressure falls below 
that of pure water and decreases along the curve A, the volume 
decreasing simultaneously. There is no actual break at O where 
the gel begins to cloud. The volume does not change much 
after reaching O, and the loss of water along the curve AaG 
causes the capillaries to fill up with air, the gel becoming cloudy. 
At O,, the capillaries are filled with air except for a small amount 
of very strongly adsorbed water and the gel is clear again. 


Saturated Water-Vapor 0; 






Pressure 


Grams Water per Gram Silica 


Fig. 11.—Vapor pressure diagram for hydrous silica. 


Along Aa, the last trace of adsorbed water is driven off. If the 
dehydration is stopped at some point along the curve A@ and the 
gel is subsequently subjected to a higher partial pressure of 
water vapor, the hydration is not reversible, but a curve Zy is 
obtained. ‘This is because the gel shrinks along AB and as it does 
not swell to any marked extent, the water is not taken up under 
the same conditions. The Z curves represent reversible phenom- 
ena at least until they cut the curve A@. If dehydration is 
stopped at any point along OOs, hydration curves like Zy and 
Zy are obtained which usually meet in the point Oe. From Oz 
to O3 and from Oz to O, the pressure-concentration curves are 


HYDROUS OXIDES OF SILICON AND GERMANIUM 177 


reversible. It is possible to pass along the path 0010.0 as often 
as one pleases but only in the one direction indicated. The 
existence of this hysteresis loop was confirmed by Anderson 
with the systems gel-water, gel-alcohol, and gel-benzene. Both 
van Bemmelen and Anderson! explained the hysteresis from the 
known fact that a liquid in a capillary tube has a greater vapor 
pressure when being filled than when being emptied, as in the 
former there is a diminution of the curvature of the liquid 
meniscus, due to incomplete wetting. Zsigmondy? attributed the 
marked hysteresis to adsorbed air which prevents the capillaries 
from being wetted readily. As a matter of fact, Patrick and 
McGavack? found no hysteresis in the adsorption of sulfur dioxide 
by silica gel when special precautions were taken to remove all 
air from the system. Moreover, no hysteresis was observed in 
the adsorption of sulfur dioxide, alcohol, carbon tetrachloride, 
and benzene by a dynamic method which consists in passing a 
mixture of air and the vapor in question over the adsorbent until 
equilibrium is attained.4 On the other hand, Patrick and 
Opdycke* were unable to eliminate the hysteresis with water by 
removal of all air. They ascribed the phenomenon to an increase 
in the viscosity of adsorbed water due to the decrease in internal 
pressure brought about by capillary and surface-tension forces. 
Since the point O may represent approximately 2 mols of water 
to 1 of silica, and the point O,, approximately 1 mol of water to 
1 of silica, there is a temptation to conclude that the dehydration 
process consists in the decomposition of a hydrate. Van Bem- 
melen showed this point of view to be untenable, since the points 
O and O, do not correspond in the vast majority of cases with 2 
mols and 1 mol of water, respectively, but vary with the history 
of the sample between 1.5 and 3 with the former and 0.5 and 1 
with the latter. Moreover, one gets the same form of curves and 
optical phenomena by substituting for water such liquids as 
aleohol, benzene, and carbon tetrachloride. Van Bemmelen’s 
work has been confirmed and extended and his conclusions 
reaffirmed by a number of investigators, among whom may be 


1Z. physik. Chem., 88, 191 (1914). 

2 “Kolloidchemie,”’ 161 (1912). 

3 J. Am. Chem. Soc., 42, 946 (1920). 

4 PaTrick and OppykE: J. Phys. Chem., 29, 601 (1925). 


178 THE HYDROUS OXIDES 


mentioned Léwenstein,'! Zsigmondy,? Thiele,? Anderson,* Bach- 
mann,* Vanzette,® Lenher,’? and Behr and Urban.’ Tschermak,? 
on the other hand, champions the view that the action of hydro- 
chloric acid on mineral silicates yields definite silicic acids 
corresponding to the salts from which they are obtained. Tscher- 
mak’s conclusions from dehydration experiments were shown 
to be altogether unwarranted, by Jordis,'° van Bemmelen,!! 
Mugge,!? Serra,'? and Thiele,'* since the breaks in the composi- 
tion curves are determined by the temperature at which the 
drying takes place, the nature of the drying agent, and the age 
and history of the sample. In spite of the evidence piled up 
against the existence of definite silicic acids, people are still 
attempting to establish their identity. Thus Schwarz and 
Menner! claim to remove adsorbed water by Willstatter’s 
method of washing the gelatinous oxide with alcohol and ace- 
tone. By a suitable choice of the conditions of preparation and 
dehydration, the existence of H2SiO3, HeSizOs, HeSiz07, and 
H,4S8i30g is regarded as definitely established; and the individual- 
ity of 128i102-10H2O and 12Si0,-9H2O is believed probable. 
As a matter of fact, these observations merely confirm what 
everybody knows, that one can get a composition for a gelatinous 
body corresponding to almost any desired formula provided one 


1Z. anorg. Chem., 68, 69 (1910). 

2 ZSIGMONDY, BACHMANN, and STEVENSON: Z. anorg. Chem., 76, 189 
(1912). 

8 Dissertation, Leipsig (1913). 

4Z. physik. Chem., 88, 191 (1914). 

5 Z. anorg. Chem., 100, 77 (1917). 

6 Atti ist. Veneto, 75, 621 (1915-1916); Gazz. chim. ital., 47, I, 167 (1917). 

7 J. Am. Chem. Soc., 48, 391 (1912). 

8 Z. angew. Chem., 36, 57 (1928). 

9Z. physik. Chane 53, 351 (1905); Zentr. Moe Geol., 225 (1908); 
Z. anorg. Chem., 63, 230 (1910); 87, 300 (1914); Nowe and Rotu: J. 
Am. Chem. Soc., 19, 832 (1897); cf. HILLEBRAND: Sitzb. Akad. Wiss. Wien, 
115, 697 (1906). 

10 Z. angew. Chem., 19, 1697 (1906). 

11 Z, anorg. Chem., 59, 225 (1908). 

12 Zentr. Mineral., Geol., 129, 326 (1908). 

13 Attt accad. Lincet, 19, I, 202 (1910). 

14 Dissertation, Leipsig (1913). 

15 Ber., 67 B, 1233 (1924); 58 B, 73 (1925). 


HY DROUS OXIDES OF SILICON AND GERMANIUM 179 


chooses the conditions properly. Pascal! analyzed three types 
of hydrous silicon dioxide magnetically and found all of them to 
behave like a mixture of anhydrous oxide and water.’ 

Structure.—Silica gel consists of minute hydrous particles 
joined together into an enmeshing network which holds water 
in the fine pores or capillaries.* Precipitated silica is completely 
amorphous, giving no x-ray interference pattern. Quartz is, 
of course, crystalline, but quartz glass is amorphous. When 
precipitated gelatinous silica is heated to 1300°, interference rings 
appear, indicating a partial conversion to. crystobalite.‘ 

The properties of hydrous silica show the usual variations with 
the temperature of precipitation. The gel precipitated by 
hydrolysis of silicon fluoride at 0° is much more readily soluble 
in hydrofluoric acid and sodium hydroxide and has a much 
stronger adsorptive capacity for methylene blue than the oxide 
formed at 100°. Schwarz and Leide® consider the two oxides to 
be distinct modifications of silica, but there is no justification for 
this assumption. ‘The difference is due to the size and physical 
character of the particles, and any member of intermediate prod- 
ucts between the 0 and 100° oxide could be made by a suitable 
choice of the conditions of formation. Indeed, Schwarz and 
Leide® have studied the gradual spontaneous loss of water and 
agglomeration of oxide particles and find it to be a continuous 
process. They regard the ageing as a definite chemical condensa- 
tion from (SiO2)z to (SiOz)ez. Until there is some definite proof 
of polymerization, I prefer the more. probable assumption that 
the ageing is a physical process involving a growth of the primary 
colloidal particles with the attending decrease in specific surface 
and loss of adsorbed water. 

Although silica is classified as a non-elastic ae the freshly 
formed jelly possesses an elasticity’ of the same order of magnitude 


1 Compt. rend., 175, 814 (1922). 

2 Le CuHaTELier: Compt. rend., 147, 660 (1908). 

SCT, p: 12. 

4 Kyroupoutos: Z. anorg. Chem., 99, 197 (1900); Gross: Umschau, 34, 
510 (1920). 

5 Ber,, 53 B, 1680 (1920). 

6 Ber., 58 B, 1509 (1920); Scowarz and STOWENER: Kolloidchem. Bethefte, 
19, 171 (1924), 

7 PrasaD: Kolloid-Z., 33, 279 (1923), 


180 THE HY DROUS OXIDES 


as that of a gelatin jelly.’ Like the latter, the elasticity modulus 
varies greatly with the water content of the sample. Silica jellies 
possess the interesting property of vibrating like a rigid body 
under certain conditions.? Holmes, Kaufmann, and Nicholas? 
obtained jellies in a glass tube that gave a tone two octaves 
above middle C when the vessel was struck. If the jellies were 
prevented from touching the walls of the glass tube by coating the 
latter with vaseline, the vibration frequency was much lower 
than for similar jellies adhering to the walls. The vibration 
frequency is increased by decreasing the concentration of silica 
and by the presence of excess mineral acid, factors which increase 
the tension and thus the effective rigidity. The same factors 
increase the tendency of the jelly to synerize, thus showing that 
both vibration and syneresis have a direct relation to tension. 
Holmes believed the vibration to be transverse, the vibration 
frequency varying approximately inversely as the diameter of 
the cylinder of jelly. Prasad‘ failed to confirm these conclusions 
for gels removed from the vessel in which they were made. The 
tone emitted by a given jelly showed wide variation depending. 
on how it was held. Moreover, by applying Newton’s formula 
for the velocity of propagation of a longitudinal wave, the vibra- 
tions were shown to be longitudinal rather than transverse. 

On account of the ease with which electrolytes diffuse into 
silica jellies, a number of interesting reactions have been carried 
out in this medium. The usual method of procedure consists 
in adding one electrolyte to the silica sol before it sets, after 
which a solution of a second electrolyte is poured on the jelly 
and allowed to diffuse into the mass where interaction takes 
place. If a crystalline precipitate is formed by the reaction, the 
crystals will be much larger and better formed than if the solu- 
tions are mixed directly. In this way, Hatschek and Simon® 
prepared large gold crystals by reducing gold salts with several 


1Lnicx: Drude’s Ann., 14, 189 (1904); SHePPARD and SwEsET: J. Am. 
Chem. Soc., 48, 539 (1921). 

2 Konkel Z. physik. Chem., 12, 773 (1893). 

3 J. Am. Chem. Soc., 41, 1829 (1919). 

4 Kolloid-Z., 33, 279 (1923). 

5 J. Soc. Chem. Ind., 31, 489 (1912); Mining Eng. World, 37, 280 (1912); 
cf. HatscuEexk: Kolloid-Z., 10, 77 (1912). 


HY DROUS OXIDES OF SILICON AND GERMANIUM 181 


reducing agents; and Holmes! prepared magnificent crystals of 
a number of metals and salts. ‘The function of the jelly is to 
prevent rapid mixing of the interacting solutions, thereby avoid- 
ing rapid precipitation and the consequent formation of amor- 
phous particles or small crystals. Under certain conditions, 
reactions in jellies give rhythmic bands or Liesegang? rings of 
precipitates instead of large crystals. What actually happens 
appears to be determined in large measure by the nature of the 
jelly. Thus silver chromate forms bands in gelatin but not in 
agar, lead chromate forms bands in agar but not in gelatin; 
while neither silver nor lead chromate forms bands in silica 
jelly, although copper chromate does. The varicolored bands of 
gold in silica described by Holmes* are obtained only in changing 
light and are not true Liesegang rings; in the dark, large crystals 
only are formed.® 

Reactions in jellies are important as offering a plausible 
explanation of certain formations innature. Thus gold salts may 
have been reduced to gold in gelatinous silica which subsequently 
become quartz. Similarly, agate has probably been produced 
from gelatinous silica into which iron and other salts have diffused 
and deposited rhythmic bands.°® 

Adsorption of Gases.—The adsorption of gases by silica 
gel has been studied in great detail by Patrick and his 
collaborators. ‘The absorbent used in these investigations was 
prepared by mixing suitable concentrations of a solution of 
silicate of soda and hydrochloric acid under violent agitation. 
After setting to a firm jelly, the material was thoroughly washed 
and dried in vacuo at a temperature varying from 110 to 300°, 
the most active samples being obtained by heating at 250 and 
300° for a half hour or more.’ 


1 J. Phys. Chem., 21, 709 (1917). 

2 “‘Chemische Reaktionen in Gallerten”’ (1898). 

3 Cf. p. 167. 

4 J. Am. Chem. Soc., 40, 1187 (1918). 

5 Davis: J. Am. Chem. Soc., 44, 2700 (1922); 45, 2261 (1923). 

6 LimseGaAne: Zentr. Mineral., Geol., 593 (1910); 497 (1911); cf. Kolloid-Z., 
10, 273 (1912). 

7 Parrick and GREIDER: J. Phys, Chem., 29, 1031 (1925). 


182 THE HYDROUS OXIDES 


Adsorption isotherms for sulfur dioxide were obtained at 
varying temperatures between —80 and +100°.! The empirical 
equation of Freundlich, 


xe de 1/n 

ms kP (1) 
where x is the amount adsorbed by the mass of adsorbent m at 
pressure P, and k and n are constants, was found to hold over 
almost the entire range studied, exceptions being at points where 
the saturation pressure was appreciable.? The straight lines 


obtained by plotting logarithm S against logarithm P at various 


temperatures were separated widely. 

Patrick considers the adsorption by a porous substance, such 
as silica gel, to be a condensation in the capillaries that is inde- 
pendent of the chemical nature of the adsorbent. Capillary 
adsorbents will differ, therefore, in the extent of their total inter- 
nal volume and also in the dimensions of the pores that make 
up the internal volume. If such be the case, the form of the 
adsorption isotherm merely expresses the distribution of the 
internal volume as a function of the dimensions of the pores. 
From this point of view, it would appear logical to seek a relation 
between the volume occupied by the adsorbed gas and the equilib- 
rium pressure rather than between the weight of adsorbed gas 
and the pressure. As a matter of fact, when the logarithms of 
the volume V of liquid sulfur dioxide obtained by dividing the 
weight of adsorbed gas by the density of liquid sulfur dioxide at 
the corresponding pressure are used as ordinates, the curves are 
brought closer together. The next step is to plot logarithm V 


against logarithm of the ‘“‘corresponding pressure”’ a where 


P, is the vapor pressure of the condensed gas at the temperature 
in question. In this way it was found that greater volumes were 
taken up at lower temperatures at the same partial pressures, 
probably because the condensed phase is more compressible at 
the higher temperatures, the surface: tension being smaller.*. 


1 PatTrick and McGavack: J. Am. Chem. Soc., 42, 946 (1920). 
2Cf. Ray: J. Phys. Chem., 29, 74 (1925). 
3 Parrick and McGavack: J. Am. Chem. Soc., 42, 976 (1920). 


HYDROUS OXIDES OF SILICON AND GERMANIUM 183 


As an empirical relationship, dividing the volume of condensed 
sulfur dioxide by the value of the surface tension o raised to a 
‘fractional power gives a correction in the right direction. The 
Freundlich equation thus takes the form 


P 1/n 
v=k( 2 
_1/n @) ( ) 


o 


1 ie 
or assuming the same value of F to hold for P. and o, 


o 1/n 
ro) ; 


This equation appears to be a general one for capillary adsorb- 
ents! and has been applied by Patrick and his pupils to the 
adsorption of sulfur dioxide, butane,? benzene, carbon tetra- 
chloride, alcohol,* and ammonia,‘ after correcting for the amount. 
dissolved in the gel water.® 

Since adsorption of gases takes place above the critical tem- 
perature where no condensation to liquid occurs under ordinary 
conditions, Patrick, Preston, and Owen® studied the adsorption 


of carbon dioxide and nitrous oxide in the region of the critical 
Po\\/" 


temperature. When the equation V =k P. was applied 


to the experimental results, it was found that k at 0° was not 
equal to k at higher temperatures near the critical point. This 
variation in k was attributed to an increase in surface tension of 
the liquid in the capillaries at temperatures near the critical 
temperature, owing to capillary forces. After correcting the 
surface tension, the equation was found to apply, indicating 


; 1 
that in all cases the constants k and * depend only on the struc- 


1 Recently Munro and Johnson [J. Phys. Chem., 30, 172 (1926)] found the 
equation to hold for the adsorption of water vapor by alumina except when 
the partial pressure approaches the vapor pressure of the liquid at the 
temperature of the adsorbent. 

2 PaTRicK and Lorn: J. Phys. Chem., 29, 336 (1925). 

3’ Patrick and OppykeE: J. Phys. Chem., 29, 601 (1925). 

4 DAVIDHEISER and Patrick: J. Am. Chem. Soc., 44, 1 (1922). 

5 Cf. NEUHAUSEN and Patrick: J. Phys. Chem., 25, 693 (1921). 

6 J. Phys. Chem., 29, 419 (1925). | 


184 THE HYDROUS OXIDES 


ture of the silica gel. Since the theory applies even above 
the critical temperatures for CO, and N2O, Patrick suggests 
that the critical temperature is raised in the pores of the gel. 

Although the most active gel is obtained by heating in vacuo 
to 250 to 300°, the oxide calcined at 1000° still adsorbs more, 
water than charcoal up to 70 per cent humidity. The cutting 
down of adsorption by calcining at 1000° is due in part to coales- 
cence of primary particles and in part to crystallization. By 
igniting at a high temperature, the adsorption capacity for water 
is reduced to zero.” The heat of wetting of silica gel by water is 
19.22 calories per gram of gel at 25°. It has been usual to 
attribute the heat of wetting of adsorbents by liquids to com- 
pression of the adsorbed liquids.4 Patrick, on the other hand, 
attributes the heat of wetting of silica gel to the filling up of 
the pores with water, whereby the water surface is reduced 
from its original very large value to practically zero. In support 
of this view he showed the net heat of adsorption (mean heat of 
adsorption minus heat of liquefaction) at 0° to be positive 
and to be equal to the heat of wetting, within the limits of 
experimental error.® 

Attention has been called to the absence of hysteresis in the 
adsorption of vapors other than water when care is taken to 
exclude air from the system. 

Adsorption of Liquids from Solution.—The adsorption from 
solution by silica gel was investigated in Patrick’s® laboratory for 
the following systems: formic acid, butyric acid, acetic acid, 
benzoic acid, and iodine from a series of solvents; nitrobenzene 
from kerosene; and acetic acid from carbon bisulfide throughout 
the entire range of concentration. <A few of the results are shown 
graphically in Figs. 12 and 18. Contrary to Freundlich’s’ view 
that very little adsorption would be expected to take place from 
organic solvents which have a relatively low surface tension, it is 

1 Benr and URBAN: Z. angew. Chem., 36, 57 (19238); cf. GuicHarp: Bull. 
soc. chim., 31, 647 (1922). 

2°VaAN BEMMELEN: Arch. Néerland. sci., 6, II 607 (1901). 

3 PaTRick and Grimm: J. Am. Chem. Soc., 48, 2144 (1921). 

4Lams and Coouipce: J. Am. Chem. Soc., 42, 1146 (1920). 

5’ ParricK and GREIDER: J. Phys. Chem., 29, 1031 (1925). 


6 Patrick and Jongs: J. Phys. Chem., 29, 1 (1925). 
7“ Wapillarchemie,”’ 259 (1922). 


HY DROUS OXIDES OF SILICON AND GERMANIUM 185 


evident that adsorption does take place to a very marked degree 
and that the amount adsorbed bears no relation to the surface 
tension of the solvent. Thus, the greatest adsorption of acetic 
acid occurs from benzene solution, and becomes less and less 
from the following solvents in order: carbon bisulfide, gasoline, 
carbon tetrachloride, toluene, and nitrobenzene; while the surface 


Millimols Adsorbed per Gram of Gel. 





Equilibrium Concentration, mols per liter 


Fia. 12.—Adsorption of acetic acid from various solvents by silica gel. 


tensions of these substances, respectively, are: 32, 15, 25, 29, 43. 
The same order of solvents holds in the adsorption of the other 
acids investigated. 

In general, Patrick finds the adsorption of a solute to increase 
as its solubility in the solvent decreases. For example, the 
adsorption of benzoic acid from the several solvents is in inverse 


186 THE HYDROUS OXIDES 


order of its solubility in these solvents. Similarly, formic acid 
is much more strongly adsorbed from toluene than is the more 
soluble butyric acid; iodine is adsorbed to a small extent in 
accord with the same laws. Moreover, nitrobenzene is adsorbed 
to a very great extent from kerosene with which it is only partially 


a 


Millimols Adsorbed per Gram of Gel. 





0 0.5 1.0 5 


Equilibrium Concentration,mols per liter 


Fic. 13.—Adsorption by silica gel of (1) nitrobenzene from kerosene and of 
benzoic acid from (2) carbon tetrachloride (3) kerosene (4) benzene (5) chloro- 
form. 


miscible, while benzene, which is much closer to kerosene in the 
solubility series, is adsorbed to a considerably smaller extent. 

In a system of V-shaped capillaries such as silica gel is assumed 
to be, the magnitude of the radius of curvature of a liquid surface 
adsorbed in the capillaries depends on the amount of liquid 


HY DROUS OXIDES OF SILICON AND GERMANIUM _ 187 


adsorbed. As the capillary forces are stronger the smaller 
the capillary, a minute amount of liquid adsorbed means that 
the liquid surfaces are very concave. Patrick concludes that 
adsorption by silica gel is due to a phase separation in the capil- 
laries caused by preferential wetting of the pores by the solute, 
followed by the production of highly concave surfaces of solute 
which effects a lowering of solubility of solute in the solvent. 
For example, when a solution of acetic acid in sulfur dioxide is 
brought in contact with silica gel, the acetic acid preferentially 
‘wets the gel, the pores of which fill up with a phase rich in acetic 
acid owing to the marked concave curvature that this phase 
presents to the body of the solution. In other words, although 
acetic acid is miscible with carbon bisulfide in all proportions 
when the surfaces are plain, this is not the case if the curvatures 
of the separating surfaces are sufficiently concave. 

This interpretation of adsorption from solution is analogous to 
the mechanism proposed by Patrick to explain adsorption from 
the gas phase. In the latter, the adsorption or condensation in 
capillaries at pressures lower than the saturation pressure at 
the given temperature is accounted for by assuming that the 
pores of the gel are presented to the main body of the gas. 
The empirical formula (38) which satisfactorily explains the 
adsorption of gases by silica gel has been changed in order to 
apply it to adsorption from solution.! The modified equation is 


1/n 
m(2) , 


where V is the volume of liquid solute adsorbed per gram of gel, 
o is the interfacial tension, S the equilibrium concentration of 
the solute in the surrounding solvent and S, the “solubility.” 
Since S, is analogous to P, in Kq. (3), it might be taken to repre- 
sent the ordinary maximum solubility of the solute in the solvent 
at the given temperature. But if this were true, S, would be 
infinity for completely miscible liquids, whereas it has been 
found always to have a finite value. By applying Eq. (4) to 
adsorption from solution, in several cases S, has been calculated 
to be always less than the ordinary solubility. S, is, therefore, 


1 Parrick and EserMman: J. Phys. Chem., 29, 220 (1925). 


188 THE HYDROUS OXIDES 


defined as the dissolving power of the solvent as uninfluenced by 
molecules of the solute subsequently entering. 

It should be pointed out that the capillary theory of adsorption 
is strictly applicable only in case there is not a specific adsorption 
factor in addition to the purely capillary phenomenon. ‘Thus in 
the case of activated carbon the chemical polarity of the adsorb- 
ent cannot be disregarded and the results cannot be interpreted 
solely in terms of the physical nature of the capillaries.! More- 
over, the adsorption of solids from solution does not come 
within the scope of the capillary theory. Finally, Chaney - 
remarks: 


Attention might be called also to the fact that when Dr. Patrick 
rationalized his vapor-adsorption curves by calculating the liquid 
volumes of his adsorbed vapors, the success of this operation was prob- 
ably due to the fact that the limiting factor in the adsorption of a 
given vapor was the total available capillary volume. In this case the 
total adsorptive capacity for the various gases would necessarily cor- 
respond with the specific volumes of the latter in liquid state, regard- 
less of whether the condensation was caused by specific polar forces or 
by capillary depression of the vapor tension. If this correctly states 
the case, the rationalization of data affected by these calculations 
throws no light at all upon the question of whether the forces operating 
to cause the adsorption were primarily capillary or chemical. 


Adsorption of Solids from Solution.—The adsorption of sodium 
hydroxide from solution by silica gel follows the Freundlich 
adsorption equation, and the adsorbed sodium ions are replace- 
able by heavy metal cations that give insoluble oxides, in accord 
with ordinary stoichiometric laws.” Patrick considers this 
adsorption to be chemical adsorption in contradistinction to 
molecular layer or capillary adsorption. ‘That is, he believes 
the molecules of sodium hydroxide combine with the colloidal 
particles of silica in the ordinary chemical sense, giving a product 
that would be a definite silicate if it were not for the magnitude 
of the colloidal particles. ‘This view fails to accounts for the 

1CHANEY: Trans. Am. Electrochem. Soc., 36, 91 (1919); Trans. Am. Inst. 
Chem. Eng., 25, Part 1, 292 (1923); Cuanry, Ray, and St. Joun: Ibid., 
25, Part 1, 309 (1923); Witson: Phys. Rev., 16, 8 (1920); See Coolidge [.J. 
Am. Chem. Soc., 48, 1795 (1926)] for criticism of the capillary theory of 


adsorption. 
2 Parrick and Barcuay: J. Phys. Chem., 29, 1400 (1925). 


HYDROUS OXIDES OF SILICON AND GERMANIUM 189 


observed increase in the ratio of alkali to silica as the concentra- 
tion of alkali solution increases. To get around this difficulty, 
Patrick suggests that the alkali may peptize the gel. This 
seems altogether improbable with an aged gel and with alkali 
concentrations of tenth normal or less, at 20°. A more probable 
assumption is that the attractive forces, whatever may be their 
exact nature,' between gel and alkali become more nearly 
saturated the greater the concentration, up to a certain point. 
Certainly one may get adsorption from solution where chemical 
union between adsorbent and adsorbate, in the ordinary sense of 
the term, is impossible or improbable. Moreover, it is possible 
to have adsorption in a definite stoichiometric ratio with the 
formation of a product having properties entirely different from 
the definite compound of the same composition.? 

Applications.—Silica gel has several desirable properties which 
render it of value as a technical adsorbent. First of all, it 
possesses a strong adsorptive capacity for vapors and liquids.’ 
Moreover, it is quite inert and the adsorbed liquids can be driven 
off and recovered if desired, simply by heating the gel, leaving 
the latter reactivated and ready to use over again.* Probably 
the most important technical applications of the adsorbent are 
in the refining of petroleum and in the recovery of benzine and 
other volatile constituents from coal gas. 

Compounds containing sulfur constitute one of the chief 
undesirable impurities in crude-petroleum distillates. These 
are ordinarily taken out by the sulfuric acid treatment which 
has the great disadvantage of removing perfectly good unsatu- 
rated hydrocarbons. Because of its specific capacity to adsorb 
the sulfur compounds, silica gel has been used with some success® 
for removing them from gasoline, kerosene, and lubricating oils. 
In the experimental plant at Baltimore, the gel powdered to 
about 200 mesh is agitated with the oil and then filtered on a 
continuous rotary filter. This process is carried out three times, 


1Cf. Lanemuir: J. Am. Chem. Soc., 38, 2221 (1916); 39, 1848 (1917); 
BarTELu: J. Phys. Chem., 28, 992 (1924). 

2 Cf. GILBERT: J, Phys. Chem., 18, 586 (1914). 

3 Parrick, Lovetacr, and Miuusmr: U.S. Patent 1335348. 

4 Cf. Taytor: Chem. Met. E'ng., 28, 805 (1923). 

5 The silica gel refinery of the Royal Dutch Shell Company at New 
Orleans, La., has been running intermittently since the summer of 1924. 


190 THE HYDROUS OXIDES 


the gel and the oil moving in countercurrents. The spent gel 
containing oil and impurities passes to a multiple-hearth muffle- 
heated activator where the oil is driven off and condensed. The 
gel has to be given a further treatment to burn off all impurities, 
after which it is used over again.! Since the adsorptive capacity 
of the gel varies for different sulfur compounds,” the actual pro- 
cedure will doubtless vary in different cases depending on the 
nature of the crude and the impurities present. It is claimed 
that desulfurized gasoline or kerosene, as prepared by this process, 
passes all specifications and has the added advantage of retaining 
the unsaturated hydrocarbons. The process eliminates the 
rerun distillation following the usual chemical treatment, 
increases the yield, and is said to give a superior gasoline or kero- 
sene. I am told, however, that the process has not proved 
successful in the commercial refining of cracked gasoline, since 
there is not a complete removal of the gum-forming compounds 
produced by cracking. 

As the adsorption process removes sulfur compounds without 
taking out the unsaturated hydrocarbons, it was possible to 
demonstrate that the former are responsible for most of the soot 
and smoke sometimes obtained in a kerosene flame. Gasoline 
purified by silica gel is said to give less carbon than ordinary 
gasoline; and the same is true for motor benzol.* Moreover, 
certain gel-refined lubricating oils are reported to give only 
about one-half as much carbon as the best ordinary oils, and 
about one-fifth as much as the poor grades, when used with the 
best grade of gasoline. 

Silica gel may be used to adsorb from natural gas the low- 
boiling gasoline vapors* which are subsequently recovered and 
blended with refinery gasoline to increase the volatility of the 
latter. It may be employed also for estimating the easily con- 
densable hydrocarbons in natural gas and coal gas. For these 


1Miuuer: Trans. Am. Inst. Chem. Eng., 15 (1), 241 (1923); Oil Gas J., 
23, 104, 151, 158 (1924); Silica Gel Corporation, Bull. 4 (1923); cf. Hot- 
LEMAN: Chem. Weekblad, 21, 187 (1924). 

2 WATERMAN, PrRQuUIN, BoGcarrs, and Goris: Chem. Weekblad, 22, 378 
(1925); WATERMAN and PErRQuin: Bernstoff-Chem., 6, 255 (1925). 

3 FinLpNER and Jones: Bur. Mines, Serial 2517 (1923); Chem, Met, Eng., 
29, 543 (1923). 

4 BURRELL: Chem, Met, Eng., 29, 548 (1923), 


HYDROUS OXIDES OF SILICON AND GERMANIUM 191 


purposes activated carbon appears to be a more satisfactory 
adsorbent.! Furness? and Williams? claim that silica gel is 
superior both to activated carbon and to oil absorbents for the 
recovery of benzine and motor spirit from coke-oven gas;* but 
this is disputed by Urbain® who reports that charcoal has great 
superiority over silica gel as a selective adsorbent of hydrocarbon 
vapors especially if they are considerably diluted, as is usually 
the case. — 

The use of silica gel has been proposed for effecting economies 
in the lead-chamber process for the manufacture of sulfuric 
acid.6 The function of the gel is to recover sulfur dioxide and 
oxides of nitrogen that are ordinarily wasted. It is also recom- 
mended for use as the carrier for platinum in the contact sul- 
furic acid process.’ 

Adsorption by silica gel is suggested as a method for recovering 
the oxides of nitrogen in the arc process for the fixation of nitro- 
gen. The removal is complete from rapid air currents at low 
concentration, and by heating the gel the adsorbed oxides may 
be recovered ready for liquefaction or for adsorption in water 
to give concentrated nitric acid. 

Because of its strong desiccating action, the gel may be 
employed in drying air for blast furnaces and to take up water 
vapor from the rapid vaporization of the liquid in the vacuum 
refrigeration process.° It is also suggested for use as a powder to 
adsorb perspiration. For this purpose it serves the double 
function of taking up the moisture and of adsorbing the odorous 
substances given out by the pores of the body while performing 
their natural functions.° 


1 Krocu: Petroleum Z., 20, 732 (1924); cf., however, Stncmr: Ibid., 20, 
279 (1924); GREEN and Waw: Colliery Guardian, 128, 88 (1924). 

2 Chemistry and Indusiry, 42, 850 (1923). 

3 J. Soc. Chem. Ind., 487, 97 (1924). 

4Lumnis: U. 8. Patent 1336360. 

5 Gas J., 167, 449 (1924); cf. Cuanny: J. Ind. Eng. Chem., 15, 1244 (1923). 

6 Parrick: U. S. Patent 1297724; Miuuter: Chem. Met. Eng., 23, 1155, 
1219, 1251 (1920). 

7 Patrick: U. S. Patent 1297724; cf. LarsHaw and Reyerson: J. Am. 
Chem. Soc., 47, 610 (1925). 

8 DantELs and McCo.tuvum: J. Ind. Eng. Chem., 15, 1173 (1923). 

9 FuLton: Chem. Age, 31, 521 (1923), 


192 THE HYDROUS OXIDES 


As a filtering agent, a refined silica known as ‘“Filtrol’’ is 
said to be three times as good a decolorizer for vegetable oils as 
is the standard fuller’s earth and is effective at a lower tem- 
perature. Besides its decolorizing property the silica adsorbs 
water, free sulfuric acid, sulfur compounds, and colloidal particles. 
The gel is also reeommended for filtering pharmaceutical prep- 
arations? since it gives more efficient results than an equal 
amount of talc, the filtering agent usually employed. 

Although silica gel is a good adsorbent for various gases, it is 
not, in general, a good catalyzer.* In the esterification of acetic 
acid with alcohol, however, it has been found to be twice as 
active as titania, the best catalyst previously known for this 
reaction. It is also an efficient catalyst for the alkylation of 
aniline ;* but its effectiveness falls off rapidly in the first hour or so 
owing to the formation of aldehydes which react with aniline to 
give easily polymerized bodies. 

Since the properties of silica gel are influenced to a marked 
degree by the method of preparation, various attempts have 
been made to improve on the gel covered by Patrick’s patent.® 
Briggs® dried the unwashed gel containing sodium chloride at 300° 
and plunged it while still hot into hot distilled water. After 
washing by decantation, the gel was dried again at 300° and the 
process repeated until all the chlorine was removed. The adsorp- 
tion capacity of this gel for nitrogen. at —190° was 60 per cent 
greater than that of a good grade of adsorbent charcoal. Holmes? 
added dilute ferric chloride to water glass until neutrality was 
reached, dried the gelatinous precipitate, and then dissolved 
out the hydrous ferric oxide with hydrochloric acid, thereby 
exposing a large surface for adsorption. The product is reported 
to have an adsorption capacity 60 per cent greater than ordinary 
silica gel. Nickel chloride or other heavy metal salts may be 
substituted for ferric chloride. 


1 KeLuy: Cotton Oil Press, 7, 38 (1923). 

2 Krantz: J. Am. Pharm. Assoc., 11, 701 (1922). 

3 MuULLIGAN and Rerp: Science, 58, 576 (1921). 

4 Brown and Retp: J. Am. Chem. Soc., 46, 1836 (1924). 

5 U.S. Patent 1297724. 

6 Proc. Roy. Soc., 100 A, 88 (1921); cf. Fetus and Firra: J. Phys. Chem., 
29, 241 (1925). 

7 J, Ind, Eng. Chem., 17, 280 (1925). 


HYDROUS OXIDES OF SILICON AND GERMANIUM 198 


SILICA SOLS 


More than a century and a half ago Pott! reported the prepara- 
tion of a ‘“‘semisolution”’ of silica; but Graham is usually credited 
with the discovery of silica sol. Graham? added to 10 per cent 
hydrochloric acid, two-thirds of the amount of water glass neces- 
sary for immediate gelatinization, and dialyzed the resulting 
mixture. Starting with highly purified chemicals, Jordis* 
obtained a pure sol containing 1.5 per cent SiO. by 6 weeks 
dialysis. Further purification in a special apparatus‘ finally 
led to the separation of plates of silica from a sol containing 0.6 
per cent of SiO2. If sulfuric acid instead of hydrochloric is used 
in preparing the sol, it is impossible to remove all the sulfate by 
dialysis. A sol containing 2 to 3 mols of sodium sulfate per mol 
of silica can be concentrated by evaporation until the silica 
content is 6 to 12 per cent of the entire mass.° Well-purified 
sols are quite clear colorless liquids, exhibiting but little inhomo- 
geneity in the ultramicroscope and giving only a very slight 
depression of the freezing point. ‘The sols are negatively charged 
but are not very sensitive to the action of electrolytes.’ Unlike 
colloidal ferric oxide, silica sol containing a little chloride does 
not prevent the appearance of turbidity on adding silver nitrate. 
The negative charge on the particles is decreased continuously 
by the addition of hydrochloric acid, becoming zero and finally 
positive without precipitation taking place. The influence of 
electrolytes on the time required for silica sols to set to a jelly 
is determined by the magnitude of their precipitating or stabiliz- 
ing action on the sol.?® 


1 WatpeEn: A History of Colloidal Silicie Acid, Kolloid-Z., 9, 145 (1911). 

2 Phil. Trans., 161, 183 (1861). | 

3 Z. anorg. Chem., 34, 455; 35, 16 (1903); 44, 200 (1905); Z. Hlektrochem., 
11, 835 (1905). 

4 Jorpis: Z. Elekirochem., 8, 677 (1902). 

5 ZSIGMONDY and Heyer: Z. anorg. Chem., 68, 169 (1910). 

6 SABANEJEFF: J. Russ. Phys-Chem. Soc., 21, 515 (1889); Brunt and 
PappabDA: Gazz. chim. ital., 31, (1) 244 (1901). 

7 PappaDA: Gazz. chim. ital., 33, 272 (1903); 35, 78 (1905). 

8 L6sENBECK: Kolloidchem. Bethefte, 16, 27 (1922). 

9 WerNER: J. Am. Pharm. Assoc., 9, 501 (1920); Krocer: Kolloid-Z., 
30, 18 (1922). 


194 THE HYDROUS OXIDES 


At the moment of its formation from water glass, Mylius and 
Groschuff! believe that silicic acid exists as such in a molecular 
solution which passes unchanged through a dialyzing membrane; 
and that the colloidal state results from polymerization of the 
acid with the splitting off of water. It seems more probable 
that the newly formed primary particles of hydrous silica are 
too finely divided to be stopped by the membrane. ‘These 
highly dispersed particles then coalesce to form larger primary 
particles with the loss of adsorbed water as a result of the decrease 
in specific surface. This ageing process goes on continuously” 
approaching crystalline anhydrous 8102 as a limit. 

Silica sol may be prepared by hydrolysis of methyl silicate* 
and of silicon sulfide, chloride, and fluoride.* It is obtained also 
by electrolyzing a solution of sodium silicate with a mercury 
cathode.> Recently Schwarz! peptized a fresh silica gel with 
ammonia and removed the excess peptizing agent in a vacuum 
desiccator containing sulfuric acid. All of these preparations 
are similar to the sol formed by Graham’s method. Bradfield® 
obtained a sol with somewhat different properties by washing 
gelatinous silica with the supercentrifuge until it was practically 
free from electrolyte. If the washing is repeated a sufficient 
number of times, the hydrogen ion concentration of the sol 
becomes constant at pH = 6.5, whether approached from the 
acid or alkaline side. Hardy’ attributes this slight acidity to 
the ability of certain of the adsorbed water molecules to ionize. 
The highly purified sol can be concentrated on the water bath to 
a syrupy consistency which can be brought back to the original 
sol condition by adding water. If the boiling is carried too far, 
minute crystals of hydrous silica separate from the sol. Even 

1 Ber., 39, 116 (1906). 

2 Cf. Schwarz and ST6wWENER: Kolloidchem. Bethefte, 19, 171 (1924); 
Scuwarz and Leipe: Ber., 58, 1509, 1512, 1680 (1920); ScHwarz and 
LeonarpD: Kolloid-Z., 28, 77 (1921); GrunpMANnN: Kolloidchem. Beihefte, 
18, 197 (1923). 

3 GRIMAUX: Compt. rend., 98, 1484 (1884). 

4 Espier and Frevitner: Ber., 44, 1915 (1911). 

5 Kolloid-Z., 34, 23 (1924). 

6 J. Am. Chem. Soc., 44, 965 (1922). 

7 J. Phys. Chem., 30, 262 (1926). 

8 Cf. BACHMANN: Z. anorg. Chem., 100, 1 (1917); Zstamonpy-SpPEarR: 
‘‘Chemistry of Colloids,’ 137 (1917); Schwarz and St6wENER: Kolloid- 
chem. Beihefte, 19, 171 (1924), 


HYDROUS OXIDES OF SILICON AND GERMANIUM 195 


the most concentrated sols show no tendency to gel, probably 
because the secondary aggregates have been broken up by 
repeated centrifuging and repeptization, leaving small groups 
of primary particles that entangle relatively little water. 

Lenher! prepared silica sol by grinding Ottawa sand for several 
days until the particles are less than 0.004 millimeter in diameter. 
When such finely divided silica is heated with an excess of water 
in a pressure bomb at 300 to 450°, gels are formed containing 15 
to 18 per cent of water. Ray? claims that crystalline quartz 
is partly converted into amorphous silica by prolonged grinding; 
but the claim appears to be without experimental foundation. 

Colloidal silica has been recommended for the treatment of 
pulmonary tuberculosis. It is administered along with protein 
in the form of tablets or better by subcutaneous or intramuscular 
injection.* Great care must be taken not only in the preparation 
of the sol® but in its administration. The treatment appears 
to be of questionable value.’ 

Kramer® finds that the addition of animal or vegetable oils to 
a 0.2 per cent solution of sodium silicate gives a fine stable emul- 
sion in which many of the drops exhibit Brownian movement. 
_ The fatty acid of the oil combines with the alkali to form soap, 
liberating colloidal silica which acts as a protective colloid for 
the emulsion. The careful addition of dilute hydrochloric 
acid produces a silica gel emulsion, while the addition of lime 
water causes coagulation forming a cheese-like coagulum and a 
thin liquid. ‘Theseexperiments are said to reproduce synthetically 
the changes in the tissue which take place in tuberculosis: 
Alkali silicate forms a fat emulsion in the tissues. The hydrous 
silica in the emulsion has a strong affinity for lime which is with- 
- drawn from the blood and causes the caseation of the emulsified 


1 J, Am. Chem. Soc., 48, 391 (1921). 

2 Proc. Roy. Soc., 101A, 509 (1922). 

3 SosmMaANn and Merwin: J. Wash. Acad. Sci., 14, 117 (1924). 

4Kiun: Minch. med. Wochschr., 67, 253 (1920); Z. Tuberk., 32, 320 
(1920); Kauue: Beitr. Klin. Tuberkulose, 47, 296 (1921); GoNNERMANN: 
Z. physiol. Chem., 99, 255 (1917). 

5 Chem. Ztg., 45, 1249 (1921). 

6 Gyr and Purpy: Brit. J. Expil. Paih., 3, 75, 86 (1922). 

7 Kauiscu: Beitr. Klin. Tuberkulose, 58, 111 (1922). 

8 Kolloid-Z., 31, 149 (1922). 


196 THE HYDROUS OXIDES 


fat as in the experiments referred to above. Carbonic acid then 
acts slowly on the ‘‘silica cheese,” converting the lime into car- 
bonate, a process designated by the pathologists as calcification. 
There remains in the tissues the small amount of hydrous silica 
which served originally as the protective colloid for the emulsion. 
In line with this, Neyland found in tubercular lymph glands of 
oxen, a silica content of 0.27 gram SiOz in | kilogram of dry tissue 
while a calcified lymph gland contained 1.54 gram SiOz per kilo- 
gram of tissue. + 


SILICATE OF SODA 


The commercial ‘‘silicate of soda’’ or water-glass solutions so 
widely used as an adhesive or cementing agent, are colloidal 
solutions containing negatively charged particles of silica and 
soda stabilized by preferential adsorption of hydroxyl ion.? 
When soda ash is fused with more than one equivalent of silica, 
a glass results. If but slightly more than one equivalent is 
used, the glass may crystallize partially, giving a definite sodium 
metasilicate. Such fusions are slowly soluble in cold water 
and readily soluble in hot water; but the solubility decreases — 
as the proportion of silica increases. When the ratio is approxi- 
mately 1Na,O to 28102, complete solution is obtained with 
difficulty; and when it reaches 1Na2,O to 48102, special methods 
must be employed to effect solution. 

The commercial ‘‘silicates of soda” are not definite chemical 
individuals; but are variable systems of sodium oxide, silica, 
and water. The solution most commonly employed in this 
country consists approximately of 1Na.,O to 3.38102 To 
prepare the solution, the molten fusion of soda ash and sand is 
run into large revolving bins partially filled with water. By 
this procedure, the melt is shattered, giving a spongy mass that 
is fairly readily peptized by water. In a preparation containing 
between 18 and 35 per cent Na2O-3.358102, the ultramicroscope 
reveals myriads of particles so clearly distinguishable that they 
cannot be greatly hydrated. However, the increase in viscosity 
with increasing concentration is typical of emulsoid colloids, 


1 Kane: Beitr. Klin. Tuberkulose, 47, 316 (1921). 
2 STERICKER: Chem. Met. Eng., 25, 61 (1921). 


HY DROUS OXIDES OF SILICON AND GERMANIUM 197 


The viscosity increases only slightly with the concentration for 
low values of the latter, but rises very rapidly when the concen- 
tration reaches a critical value. The slope of the viscosity- 
concentration curve is dependent on the sodium-oxide-silica 
ratio. ‘Thus a change in concentration of but 1 per cent in a 
solution of Na,O-:3.98iO.2 causes an increase in viscosity from 
379 to 7000 centipoises; while an 8 per cent change in concentra- 
tion is necessary for asimilar increase, in a solution of NazO- 2S8iOx.. 
The rate of change of viscosity is important as a measure of the 
rate of set when the silicate is used as an adhesive. 

Gels formed by concentrating silicate sols containing a high 
percentage of silica are very elastic. Stericker! reports that 
balls of the gel dropped 40 feet will rebound two-thirds of the 
distance; and yet, like fluids, they will take the shape of the con- 
tainer in which they are placed. 

When a solution of water glass is neutralized by an acid, it 
sets to a jelly sooner or later, provided the concentration is not 
too low. ‘The speed of gelation is determined by a number of 
factors among which may be mentioned the concentration, the 
excess of hydrogen or hydroxyl ions present, the impurities, 
the kind of acid used, and the temperature.2. Holmes gives 
directions for preparing various types of jellies setting in any 
required time. 

The addition of concentrated sodium chloride to water-glass 
solutions throws down a gelatinous precipitate which tends to 
become granular and hard; an excess of brine causes repeptiza- 
tion of the gel. Malcolmson’ took advantage of this behavior to 
increase the volume of silicate solution without altering its 
viscosity appreciably. By proper adjustment of the concen- 
tration of brine, it was possible to extend the volume approxi- 
mately 21 per cent. Unfortunately, the adhesive properties of 
the extended solution are not so good as those of the original 
silicate; and the cost of mixing is usually greater than that of the 
silicate replaced.* 


1 Bogue’s ‘Colloidal Behavior,” 2, 565 (1924). 

2 FLEMMING: Z. physik. Chem., 41, 427 (1903); Houmus: J. Phys. Chem., 
22, 510 (1918). 

3 J. Ind. Eng. Chem., 12, 174 (1920). 

4 STERICKER: Bogue’s ‘Colloidal Behavior,” 2, 569 (1924). 


198 THE HYDROUS OXIDES 


The addition of iron or aluminum salts to water glass yields a 
gelatinous precipitate of variable composition containing hydrous 
silica and hydrous ferric or aluminum oxide as the case may be. 
The action between alum and water glass is made use of in the 
mineral sizing of paper.t The gelatinous precipitate is adsorbed 
by the paper, giving it a smoother and harder finish than is 
obtained in its absence. The precipitate sizes for printers’ 
ink which has an oil base; and it increases the resistance to aque- 
ous inks, possibly because of an increased retention of resin in 
the paper. 

When small crystals of various metallic salts are dropped into 
an 18 per cent solution of commercial water glass, growths 
resembling plant shoots spring up, giving rise to the so-called 
“artificial vegetation” or ‘‘colloidal forest.”? The growths are 
colored when colored salts are used, but the water-glass solution 
does not become colored except in the case of manganese salts. 
The form of the growths is different with different metals. For 
example, hair-like filaments result with cadmium salts and thick 
fungoid growths with nickel salts. The growths are tubular 
and act as semipermeable membranes. 

Applications—Commercial water glass finds application in a 
great many branches of industry. Attention has been called to 
its use in paper sizing and asan adhesive. For the latter purpose 
it is said to be the only substance employed in the fiber-container 
industry for gluing together the components of both double- 
faced corrugated board and laminated solid fiber board.* It is 
also used for impregnating sandstone and other porous stones as a 
protection against weathering. ‘This is accomplished by treat- 
ing the stone with water glass followed by the application of a 
solution of calcium or aluminum sulfate, which precipitates an 
insoluble gel in the pores of the stone, greatly increasing its 
hardness and durability. It is also employed as a cement in 
the manufacture of artificial stone from sand and lime. A 
mixture of 2 parts fluorspar and 1 part powdered glass, made into 


1VaiL: Chem. Met. Eng., 25, 823 (1921); Srmrickrer: Paper Ind., 5, 1398 
(1923). 

2 Do.uiries: Compt. rend., 143, 1148 (1906); Ross: Proc. Roy. Soc. New 
South Wales, 44, 583 (1910); cf. J. Chem. Soc., 102, II, 49 (1912). 

3 Mautcotmson: J, Ind. Eng. Chem., 12, 174 (1920). 


HYDROUS OXIDES OF SILICON AND GERMANIUM 199 


a thick paste with water glass, gives a cement for glass and 
porcelain. 

On account of its detergent properties, water glass is frequently 
added to cheap soap.! It is employed in the calico-printing and 
dyeing industry and in fixing fresco colors by the process of 
stereochromy. It is also used for rendering wood, paper, etc. 
inflammable; and to a limited extent, in preventing wood from 
rotting and in the preservation of eggs. 

Because of its peptizing or deflocculating action, silicate of 
soda may be employed to produce the clay ‘‘slip”’ from which 
the casts are made in the manufacture of pottery and sanitary 
ware. An undeflocculated slip containing around 20 per cent 
of water is a stiff plastic mass. By working into it about 0.1 
per cent each of Na,O- 3.3810. and sodium carbonate, the mixture 
becomes sufficiently fluid that it can be pumped readily through 
l-inch pipes. This casting slip is then run into plaster molds 
which adsorb the water and flocculate the clay. Silicate of soda 
or some similar agent must also be used to prepare the slip in the 
electrical casting of clay.? 

The deflocculating action of silicate of soda on siliceous and 
argillaceous material has been applied in concentrating ore by 
flotation. Sulman* found that less of the gangue constituents 
are carried into the froth, the greater their degree of dispersion. 
The importance of silicate of soda solutions in flotation is due to 
the wide range of their deflocculating action on the gangue. 
On the other hand, some ores suchas copper sulfide are flocculated, 
thereby facilitating their flotation. Another method of ore 
concentration consists in deflocculating the gangue and removing 
it from the settled concentrate by decantation.* 


THe Hyprous OXIDES oF GERMANIUM 


Germanium Dioxide——Hydrous GeO, is precipitated in a 
gelatinous form by the hydrolysis of germanium tetrachloride® 


1 Cf, RicHARDSON: J. Ind. Eng. Chem., 15, 241 (1923); SrmrickEr: [bid., 
15, 244 (1923). 

2 KLEEMAN: Phys. Rev., 20, 212 (1922). 

3 Bull. Inst. Mining Met., 29, 49 (1920). 

4 BorcHerpT: U.S. Patents 1446375 to 1446378; 1448514, 1448515 (1923). 

5 WINKLER: J. prakt. Chem., (2) 34, 211 (1886); DmnNis and JOHNSON: 
J. Am. Chem. Soc., 45, 1380 (1923). 


200 THE HYDROUS OXIDES 


or tetrabromide.'! It is also obtained by passing carbon dioxide 
into a solution of the oxide in alkali.2 It forms no hydrates,’ 
but it holds on to the last trace of adsorbed water quite strongly, 
complete dehydration requiring a temperature of 950°.4 The 
precipitated oxide is fairly soluble in water, giving an acid solu- 
tion® from which microscopic rhombic crystals® separate on 
evaporation. 

Miiller and Blank’ recognize three distinct preparations: (1) 
The hydrolyzed oxide obtained by hydrolysis of GeCl,. This 
gel forms with cold water a milky suspension which clears up on 
boiling. It is readily soluble in hydrofluoric and hydrochloric 
acids. (2) The ‘‘evaporated”’ oxide resulting from evaporation 
of the aqueous solution. ‘This preparation is but slowly soluble 
in cold water but dissolves in hot water after a short time. (3) 
The ‘insoluble’ oxide prepared by heating the evaporated 
oxide to any temperature between 200° and its melting point 
(about 1100°) and then boiling the mass thoroughly with water 
to remove the unconverted ‘‘evaporated”’ oxide. This prepara- 
tion is insoluble in water and in boiling hydrochloric and hydro- 
fluoric acids, alkali, and ammonia, but becomes soluble on fusion. 
Analogous to the behavior of different preparation of silica and 
stannic oxide, the varying properties of the three germanium 
oxides might be due to differences in size of the primary particles 
and physical character of the precipitates. Against this hypoth- 
esis, it was shown: that the yield of ‘‘insoluble” oxide formed by 
heating the evaporated oxide at different temperatures for the 
same period of time increased up to 380° and then decreased to 
the melting point. This suggests that 380° may represent the 
temperature of maximum velocity of transformation of the 
evaporated oxide into the ‘‘insoluble” form. When the time of 
heating of the evaporated oxide was varied at the constant 
temperature of 280°, the yield of the ‘‘insoluble” form increased 

1 Dennis and Hance: J. Am. Chem. Soc., 44, 299 (1922). 

2 WINKLER: J. prakt. Chem., (6) 34, 177 (1886). 

3 Van BEMMELEN: Rec. trav. chim., 6, 205 (1887). 

4 DENNIs, TRESSLER, and Hancn: J. Am. Chem. Soc., 45, 2033 (1923). 

5 WINKLER: J. prakt. Chem., (2) 34, 211 (1886); Mt.Lurr and Iszarp: 
J. Med. Sci., 163, 364 (1922). | 


6 HausHorer: Sitzb. Akad. Miinchen, 1, 133 (1887). 
7 J. Am. Chem. Soc., 46, 2358 (1924). 


HYDROUS OXIDES OF SILICON AND GERMANIUM 201 


in such a manner as to suggest that the conversion could never 
reach 100 per cent. For this reason and because the yield of the 
insoluble form varied greatly with different preparations of 
evaporated oxide, Miiller and Blank suggest that three allo- 
tropic forms of germanium oxide may exist, the evaporated 
oxide being a mixture of two of them. | 

Germanium dioxide is said to be of value in the treatment of 
secondary and pernicious anemia.! 

Germanous Oxide.—Unlike silicon but like tin, germanium 
forms an ‘‘ous”’ oxide. ‘This is precipitated in a gelatinous form 
by the action of alkalies on a solution of GeCl, or by the hydrolysis 
of germanium chloroform, GeHCl;. When thrown down in the 
cold, the precipitate is yellow but it becomes yellowish red by 
boiling in the mother liquor. It is peptized by boiling water, 
giving a yellow sol. According to Hantzsch,? it is very slightly 
soluble in water, acting as a weak monobasic acid of the constitu- 
tion HGeO: OH, analogous to formic acid. 


1Lenker: Penn. Med. J., 26, 86 (1922); Kast, Crouti, and Scumitz, 
J. Lab. Clin. Med., 7, 643 (1922); MititumrR and Iszarp: J. Med. Sci., 163, 
364 (1922); J. Metabolic Research, 3, 181 (1923); cf., however, Minor and 
Sampson: Boston Med. Surg. J., 189, 629 (1923). 

2Z. anorg. Chem., 30, 289 (1902). 


CHAPTER VIII 
THE HYDROUS OXIDES OF TIN AND LEAD 


HyprRous STANNIC OXIDE 


‘As early as 1812 Berzelius' called attention to differences 
between the hydrous oxide formed by precipitation of stannic 
chloride with alkali and the product resulting from the action of 
nitric acid on tin. Berzelius thought at first that he was dealing 
with two degrees of oxidation; but this was disproved by subse- 
quent investigations of Davy, Gay Lussac, and Berzelius? himself. 
Thus, Berzelius was led to conclude that the two preparations, 
having widely different properties, were simply modifications of 
the same oxide. In this way the term isomer or isomeric modifica- 
tion was introduced in chemistry.® 


PRECIPITATED HYDROUS STANNIC OXIDE 


Since the oxides formed by precipitation of stannic salts and 
by the action of nitric acid on tin both give a very slight acid 
reaction when shaken with water, they are commonly designated 
as orthostannic and metastannic acids, respectively. The 
earlier chemists regarded them as distinct chemical individuals 
and recognized the similarities and differences between the two 
that are listed in Table XVII. In the light of what is now known 
of the colloidal state of matter, the statements of earlier chemists 
concerning the properties of these bodies are inaccurate in many 
respects. Since both substances are more properly termed 
hydrous oxides rather than acids, I shall designate them as alpha 
. oxide and beta oxide, respectively, instead of as orthostannic 
and metastannic acid. 


1“Tehrbuch,” 5th ed., 2, 596 (1812). 

2 Ann. chim. phys., [2] 5, 149 (1817). 

3 Apeaa@: ‘Handbuch anorg, Chemie,”’ [2] 3, 593 (1909), 
202 


THE HYDROUS OXIDES OF TIN AND LEAD 


TaBLe XVII 


203 





Orthostannic acid 


Metastannie acid 


Preparation.......| Precipitation from solu-| Action of concentrated 
tion of stannic salt HNO; on tin 
Lin ECW aS ae H.SnO; H.SnO; 
Action of HNO;...{ Easily soluble Insoluble 
Action of HCl..... Easily soluble; not precipi- | Insoluble. Product treated 
: tated by excess acid with concentrated acid 


and filtered dissolves in 
water but precipitates 
again with excess acid 
Insoluble but swells in con- 
centrated acid, forming a 
mass that is soluble in 
water 

Soluble when freshly pre- 
pared; precipitated by 
excess alkali 
Yellow precipitate 
solution in HCl 


Action of H2SO,...| Easily soluble 


Action of caustic | Easily soluble; not precipi- 
alkalies. tated by excess alkali 


from 


Action of SnCl,....| No action 


Formation.—The typical a oxide is prepared by precipita- 
tion of SnCl, or SnBr, with alkali! or with an excess of the car- 
bonate of barium or calcium;? and by precipitating a solution of 
soluble crystalline stannate having the formula M.Sn(OH),' 
with mineral acid.* Rose® claimed to get the a oxide by hydrol- 
ysis of a dilute solution of SnCl, at the boiling point. This is 
unquestionably incorrect, since it has been observed repeatedly 
that a oxide, formed by hydrolysis of SnCl, at low temperatures, 
goes over to @ oxide gradually on standing or very rapidly at 
the boiling point. Similar observations have been made with 
SnBr,’ and with Sn(NO3),. Lorenz® obtained the a oxide by 

1 BerzeEvivs: “Lehrbuch,” 5th ed., 2, 1596 (1812). 

2 Scuirr: Liebig’s Ann. Chem., 120, 47 (1861). 

3 BeLLucci and ParRravaNno: Z. anorg. Chem., 45, 142 (1902). 

4Fremy: Ann. chim. phys., (3) 12, 463 (1844); 23, 385 (1848); Kita: 
Pharm. Zig., 538, 49 (1908). 

5 Pogg. Ann., 75, 1 (1848). 

6 BarrorD: J. prakt. Chem., 101, 368 (1867); EncEL: Compt. rend., 124, 
765 (1897); 125, 464, 651, 709 (1897); Zstamonpy: Liebig’s Ann. Chem., 


301, 368 (1898). 
7 Lorenz: Z. anorg. Chem., 9, 371 (1896). 
8 Z. anorg. Chem., 12, 436 (1896), 


204 THE HYDROUS OXIDES 


electrolyzing an alkali chloride, nitrate, or sulfate solution using 
a platinum cathode and a tin anode. 

The typical 6 oxide is prepared by the oxidation of tin with 
moderately concentrated HNO3. Weber! claimed that acid of 
1.2 sp. gr. gave both a and B oxides, while acid of 1.35 sp. gr. 
produced a clear solution from which @ oxide was obtained by 
warming. Hay? and Scott’? likewise observed the complete 
dissolution of tin in moderately dilute nitric acid (1:1) at 2°, 
from which 8 oxide precipitated by warming or by standing at 
ordinary temperatures. ‘The solution contained stannous nitrate 
stannic nitrate,* and doubtless colloidal stannic oxide® in varying 
amounts depending on the concentration of acid and the tem- 
perature. As before noted, 8 oxide is produced whenever a 
dilute solution of a crystalline tin salt undergoes hydrolysis at 
the boiling temperature. A solution of amorphous sodium meta- 
stannate, so called, likewise precipitates 6 oxide when heated. 

From this survey, it is evident that either oxide may be pre- 
pared by hydrolysis of stannic salts under suitable conditions. 
In all probability the first product of this hydrolysis is always a 
oxide, which subsequently goes over to 8 oxide quite slowly at 
ordinary temperatures but with increasing rapidity as the tem- 
perature is raised. ; 

Composition.—By drying different precipitated oxides under 
the proper conditions, earlier investigators have reported the 
preparation of a wide variety of supposedly definite hydrates 
and hydrated acids. An extensive study of the change in vapor 
pressure of different preparations with the temperature led van 
Bemmelen to conclude that such compositions were purely acci- 
dental, depending on the method of formation, the method of 
drying, the temperature, and the age of the sample. Van 
Bemmelen’s observations were confirmed and extended, and his 


1Pogg. Ann., 122, 358 (1864). 

2 Chem. News, 22, 298 (1870). 

3 Chem. News, 22, 322 (1870). 

4 WaLkER: J. Chem. Soc., 68, 845 (1893). 

5 MECKLENBURG: Z. anorg. Chem., 64, 370 (1909). 

6 Fremy: Ann. chim. phys., (3) 12, 463 (1844); 23, 393 (1848); cf. WEBER: 
Pogg. Ann., 122, 358 (1864); cf. Granam: Liebig’s Ann. Chem., 18, 146 
(1835); Scuarrner: Jbid., 61, 168 (1844); CaRNELLEY and WALKER: J. 
Chem. Soc., 58, 83 (1888). 


THE HYDROUS OXIDES OF TIN AND LEAD 205 


conclusions reaffirmed by Lorenz' and Mecklenburg.? Recently, 
however, Willstatter and his collaborators? adopted the older 
view that the behavior of the variety of oxides could be explained 
best by assuming the existence of more or less stable hydrates. 
Willstatter claimed to remove all the adsorbed water from a 
compound by drying rapidly in vacuum or by leaching with 
acetone. The composition of a gel formed in a special way and 
dried by the acetone method at —35° to +10° was represented by 
the formula Sn(OH),: HO; but when dried at room temperature 
the analysis showed a composition Sn(OH),4, which was regarded 
as the first member of a series of a stannic acids. In an aqueous 
medium, Sn(OH).4 was supposed to go over into other less basic 
members of the series. Thus by suitable conditions of precipi- 
tation and drying with acetone at 0 to 10°, orthodistannic acid 
was supposedly formed; at 35 to 46°, orthotristannic acid; and 
soon. Different so-called 8 stannic acid were likewise prepared 
and many of them assigned formulas. 

As proof of hydrate formation, Willstitter cites the regions of 
almost constant water content in the temperature-composition 
curves of acetone-dried preparations. Such evidence is alto- 
gether inconclusive, particularly when the nature and location 
of the “flats” in the curves are determined almost exclusively 
by the history of the sample. The same may be said of the 
‘flats’ in the temperature-vapor-pressure curves of van Bemme- 
len. The adsorptive capacity of a hydrous oxide for water at 
different stages of dehydration is determined by the physical 
character of the preparation; hence a “‘flat”’ corresponding to a 
definite hydrate is purely accidental and can be duplicated only 
by following a set method of procedure in precipitation, ageing, 
and drying. Willstatter’s comparison of the behavior of hypo- 
thetical high-molecular hydrated stannic acids with their groups 
Sn:O and Sn: OH, to that of carbohydrates with their groups 
C:O and C- OH, appears highly fantastic and illusionary. 

Action of Acids.—F reshly prepared a oxide is readily soluble in 
dilute HCl and is not precpitated by an excess of acid even at 
the boiling point; whereas 6 oxide is insoluble in both dilute and 

1Z. anorg. Chem., 9, 369 (1895). 


2Z. anorg. Chem., 64, 368 (1909); 74, 207 (1912); 84, 121 (1914). 
8 WILLSTATTER, Kraut, and Fremery: Ber., 57 B, 63, 1491 (1924). 


206 THE HYDROUS OXIDES 


concentrated acid. However, if @ oxide is treated with concen- 
trated HCl, a gelatinous mass is formed which Engel! believes 
to be a salt, metastanyl chloride. This product is taken up by 
water but is reprecipitated by boiling or by adding concentrated 
HCl. In view of the variety of acids that are supposed to be 
derived from stannic oxide, it is not surprising to encounter a 
number of basic salts of tin. Thus Engel claims to get SnCh, 
SnsOsCl.:4H.O, and Sn;O9Cle:2H2O corresponding to his ortho, 
meta, and para acids.2_ While SnCl, is obtained by the action of 
concentrated HCl on the @ oxide* and H2SnCl,* is formed by 
passing gaseous HCl into a solution of stannic chloride, there is 
little or no evidence to support the view that the amorphous 
precipitates, obtained from solutions of a@ and 8 oxide under 
varying conditions, are definite compounds. Van Bemmelen® 
was the first to recognize the real nature of such solutions. He 
proved it to be incorrect to speak of “solubility” of the oxides 
in acid by showing that the acid which holds the a oxide in what 
was thought to be a true solution may be neutralized almost 
entirely without the oxide precipitating; that the salt formed 
may be removed by dialysis without precipitation taking place; 
and finally, that the solutions may be boiled, converting a oxide 
into B, which likewise does not precipitate unless the boiling is 
continued too long. Van Bemmelen also observed the adsorp- 
tion of HCl by both oxides. Below the concentration which 
causes peptization, the adsorption isotherms have the usual 
form, indicating that the amount adsorbed depends on the 
concentration of acid in contact with the oxide. The adsorption 
was found to be less with 6 oxide than with a, and the older and 
denser the 8 oxide, the less was the adsorption. 

The action of hydrochloric acid on the different oxides can now 
be explained. The newly formed oxide possesses a softer and 


1 Compt. rend., 724, 765 (1897); 125, 464, 657, 709 (1897). 

2 Cf. TSCHERMAK: J. prakt. Chem., 86, 334 (1862); Mauuet: J. Chem. Soc., 
35, 524 (1879); ScomurER-KeEstnER: Ann. chim. phys. (3) 58, 471 (1860); 
Orpway: Am. J. Sci., (2) 28, 220 (1857); cf. also Rosz: Pogg. Ann., 75, 1 
(1848); WirrstrIn: Jahresber., 1850, 321. 

3 Barrorp: J. prakt. Chem., 101, 368 (1867). 

4 Kowatwsky: Inaugural Dissertation, Breslau (1902). 

5 “Die Absorption,” 56, 393 (1910); Z. anorg. Chem., 23, 111 (1900). 

6 GRAHAM: Liebig’s Ann, Chem., 121, 1 (1861). 


THE HYDROUS OXIDES OF TIN AND LEAD 207 


looser structure than the 6 oxide, and so the former is readily 
peptized by dilute acid and the colloid is stable even in the pres- 
ence of a very small amount of acid. A high concentration of 
acid converts it into a true solution of SnCl. The coarser, 
denser particles of @ oxide are insoluble and are not peptized by 
dilute HCl. Concentrated acid, on the other hand, peptizes 
the oxide; and if the excess of acid is poured off, the particles 
will go into solution in water (dilute hydrochloric acid) from 
_ which they are precipitated by excess acid or by boiling. Since 
a oxide changes to @ even at ordinary temperatures and in contact 
with water, we should expect the colloidal solution of a oxide 
formed by hydrolysis of a dilute solution of SnCl, gradually to 
assume the properties of the dilute hydrochloric acid solution of 
8 oxide, as observed by Fremy, Rose, L6wenthal,! and others.? 

In the light of the experiments of van Bemmelen, it is unlikely 
that the amorphous masses obtained by Engel and others are 
definite chlorides. Further doubt is thrown on this by Biron,’ 
who obtained products similar to Engel’s meta and para chlor- 
ides, but found their composition to be indefinite. 

Mecklenburg investigated the properties of the oxides obtained 
by the simultaneous action of various mixtures of hydrochloric 
and nitric acid on tin. The products adsorbed both acids in 
proportion to the relative concentrations in the original solution; 
the ratio, total acid:SnOe, remaining approximately 0.5:1. 
The greater the hydrochloric acid content of the hydrous oxide, 
the more readily it was peptized by water, a circumstance which 
led Mecklenburg to attribute to this acid a protecting action 
similar to that of a protective colloid. 

Collins and Wood‘ regard the various stannic oxides as salt- 
like complexes formed by continued condensation between 
molecules of stannic hydroxide acting as acid and base, respec- 
tively. The peptization by hydrochloric acid is looked upon 
as essentially a chemical process, although the first stage in the 
process is recognized as adsorption of hydrochloric acid by the 


1 J. prakt. Chem., 77, 321 (1859). 

2 BaRFOED: Loc. cit.; ALLEN: J. Chem. Soc., 25, 274 (1872); Lorenz: Z. 
anorg. Chem., 9, 369 (1895). 

3 J. Russ. Phys.-Chem. Soc., 36, 933 (1904). 

4 J. Chem. Soc., 121, 441 (1922). 


208 THE HYDROUS OXIDES 


oxide particles in varying amounts, depending on the extent of 
surface which in turn is proportional to the degree of condensa- 
tion. Following this adsorption, a reaction is thought to take 
place between the adsorbent and the adsorbed acid, due to neu- 
tralization of the latter by some of the basic affinities of the original 
stannic hydroxide still possessed by the condensed acid. The 
resulting salt will yield a positive complex ion and chloride 
ion, if the ionization is not prevented by the presence of too high 
a concentration of chloride ion from ionization of hydrochloric 
acid. While the behavior of oxides prepared in different ways 
has been interpreted by the aid of these assumptions, there seems 
to be no real justification for postulating the existence of a wide 
variety of condensed stannic acids and complex basic salts. It 
seems much more likely, particularly in the light of the recent 
observations of Pascal! and Yamada,” that the various products 
are simply hydrous stannic oxides that have adsorbed hydrogen 
and chloride ions, the positive charge on the colloidal particle 
arising from preferential adsorption of hydrogen ion. 

Sulfuric acid acts on both a and 8 oxide in much the same way 
as HCl. Dilute HNO; peptizes a oxide quite readily when the 
latter is freshly prepared; but 6 oxide is neither dissolved nor 
peptized by even the most concentrated acid. However, 6 
oxide adsorbs HNO; to a certain degree and the adsorbed 
acid can be removed only by prolonged washing.*® 

The adsorbing power of 8 oxide for phosphoric acid deserves 
special mention, since a standard analytical procedure for sepa- 
rating this acid from mixtures consists in adding tin foil to the 
nitric acid solution, the resulting 6 oxide carrying down the 
H3PQO,.4 For the complete precipitation of 1 mol of phosphoric 
acid, 6 to 7 atoms of tin must be present according to Classen;° 
7 according to Antony and Mondolfo;® and about 13 according 


1 Compt. rend., 175, 1063 (1922). 

2 J. Chem. Soc. Tokyo, 44, 175 (1928). 

3 JORGENSEN: Z. anorg. Chem., 57, 353 (1908). 

4 Reyonoso: J. prakt. Chem., 54, 261 (1851); Roscoz and ScHORLEMMER: 
“Treatise on Chemistry,” 2, 899 (1923); Reisia: Liebig’s Ann. Chem., 98, 
339 (1856); Girarp: Compt. rend., 54, 468 (1862). 

5 “ Angew. Methoden analyt. Chem.,”’ 2, 555 (1903). 

6 Gazz. chim. ital., (2) 38, 145 (1898). 


THE HYDROUS OXIDES OF TIN AND LEAD 209 


to Wobling.t| Mecklenburg? showed conclusively that the 
removal of H3;PO, by precipitated stannic oxide was not due to 
the formation of a definite stannic pyrophosphate, but to adsorp- 
tion, the amount of acid carried down depending not only on its 
concentration but on the nature of the oxide. The adsorption 
isotherms for five oxides, prepared at different temperatures, 
showed a decreasing adsorption capacity with increasing temper- 
ature of formation. 

Action of Alkalies——Dilute caustic alkalies carry both the a 
and 6 oxides into solution. For a long time this solution was 
believed to taak place by virtue of the formation of definite alkali 
stannates and metastannates, largely because evaporation of solu- 
tions of a oxide in strong alkali yield definite crystals having the 
formula M.Sn0O3-3H.O? or M2Sn(OH)..4 However, if the solu- 
tion of the a oxide in dilute alkali is not evaporated but treated 
with alcohol, a precipitate is formed, varying in composition 
from 5 to 17 mols of SnO, to 1 mol of K,O, depending altogether 
on the relative amounts of the two substances in solution. In 
like manner, the precipitate obtained from a solution of 8 oxide 
in alkali varies very widely, depending as it does on the condi- 
tions of formation.® 

Twenty-five years ago, van Bemmelen® called attention to the 
colloidal nature of the solutions of both a and @ oxides in 
alkalies. He agitated the same amounts of a oxide with like 
volumes of cold dilute KOH of various concentrations; when the 
concentration of alkali was less than 8.8 mols of K,O to 100 of 
SnO., the peptized oxide precipitated spontaneously, carrying 
down with it a greater part of the alkali. Asis usual, the amount 


1 “Tehrbuch anal. Chem.,’”’ 405 (1911). 

2Z. anorg. Chem., 74, 215 (1912). 

3 Orpway: Am. J. Sci., (2) 40, 173 (1865); Marianac: Ann. mines, (3) 
15, 277 (1859); Mosere: J. prakt. Chem., 28, 230 (1848). 

4Beutuuccr and PaRRAvANO: Z. anorg. Chem., 45, 142 (1902). Other 
hydrates are also known; HanrriEy: Dinglers polytech. J., 144, 66 (1867); 
Jonas: Chem. Zentr., 607 (1865). 

5 Wremy: Ann. chim. phys., (8) 12, 460 (1844); 23, 385 (1848); Rosp: 
Pogg. Ann., 75, 1 (1848); 105, 564 (1858); Weper: Jbid., 122, 358 (1864); 
Muscuuus: Compt. rend., 65, 961 (1867); Mosura: Berzelius’ Jahresber., 
22, 144 (1848). 

6 “Die Absorption,” 57 (1910), 


210 THE HYDROUS OXIDES 


of KOH adsorbed by a given amount of oxide varied with the 
temperature and concentration, and there was no indication of 
the formation of definite stannates. He thus accounted for the 
wide variation in the composition of the precipitate thrown out 
by alcohol from the colloidal solution of the @ oxide in alkali, 
as observed by Ordway and others. 

Van Bemmelen obtained similar results with the more difficultly 
peptizable 8 oxide. The first action of dilute NaOH was to 
produce an opalescent solution that, in itself, showed the oxide 
to be in the colloidal state. The relative amounts of oxide and 
alkali in the solution were varied widely; and, as in the case of 
a oxide, spontaneous precipitation took place the more rapidly, 
the greater the relative amount of SnOe. These colloids were 
coagulated by excess alkali, which was not the case with colloidal 
a oxide. The precipitated oxides obtained in any case, adsorbed 
alkali in varying amounts, depending on the alkali concentra- 
tion and the physical character of the precipitate.! 

Some observations of Heinz? and of Franz® give some idea ys 
the relative ease of peptization of different oxides by alkali. 
The former prepared a colloidal solution of an a oxide in which the 
ratio K,0: SnO, was 1:200; and the latter obtained colloidal B 
oxides in which this ratio varied from 1:25 to 1:50. As is usual, 
the peptizing action of potassium hydroxide is greater than that of 
sodium hydroxide, the precipitating power of K’ ion being appre- 
ciably smaller than that of Na’ ion. 

Mordanting Action.—Tin salts, particularly SnCl, are some- 
times used as mordants in dyeing cotton, wool, and silk. The 
salt is adsorbed by the dye fiber and subsequently hydrolyzes, 
giving hydrous stannic oxide which forms lakes with certain 
dyes that are distinguished by their brilliancy.* Vignon® 
studied the action of both 6 and a oxide on a basic dye, pheno- 
safranine; with the former, a brilliant-red lake was formed, while 
with the latter no dye was taken up. Thus the dye is readily 

1Cf. WinTGEN: Z. physik. Chem., 108, 238 (1923). 

2 Dissertation, Géttingen (1914). 

3 Dissertation, Géttingen (1913). 

4WeisER: J. Phys. Chem., 26, 424-427 (1922); cf. Cottins and Woop: 
J. Chem. Soc., 121, 2760 (1922). 


® HerzFeLp: ‘‘Das Farben und Bleichen der Textifasern,” 1, 73, (1904). 
6 Compt. rend., 112, 580 (1891), 


THE HYDROUS OXIDES OF TIN AND LEAD 211 


adsorbed by the loose finely divided particles of a oxide, while 
the larger denser particles of 8 oxide have comparatively little 
adsorbing power.! This behavior is general and has led to the 
statement: “The formation of metastannic acid during the prep- 
aration of tin mordants is called firing; it must be avoided, since 
this substance has no mordanting power and its generation 
involves loss of tin.’’2 | 

The Question of Isomers.—A survey of the properties of the 
so-called a and 8 oxides discloses marked differences in their 
solubility, adsorbability, and ease of peptization. The typical 
a oxide is quite soluble in concentrated acids and alkalies forming 
definite salts under suitable conditions; it possesses a marked 
capacity for adsorption and is readily peptized. The typical 
6 oxide, however, is very difficultly soluble, has a comparatively 
sight capacity for adsorption, and is not peptized by dilute 
acids or alkalies. If these two oxides are definite isomers, then 
any product having properties intermediate between the two 
might be looked upon as a mixture. If the difference in the 
properties of the two, however, is due to variation in the size 
of the primary particles and the compactness of the hydrous 
mass, then any product with intermediate properties must be a 
chemical individual and not a mixture. 

Van Bemmelen came out definitely against the view that the 
difference between the two oxides is due to allotropy rather than 
to physical structure. He called attention to the absence of a 
definite inversion point at which a oxide goes over to 8, and 
demonstrated the slow but continuous transformation at ordinary 
temperatures even under water. Mecklenburg? comes to a 
similar conclusion: ‘‘The a@ and £6 stannic acids are hydrous 
oxides that are little if at all soluble in water, and differ from each 
other in the size of the particles.” 

Mecklenburg‘ prepared five distinct oxides by hydrolysis of 
stannic sulfate at 0, 25, 50, 75, and 100°. ‘These oxides were 
dried in the air and ground toa powder. Each product was differ- 


1Cf, Moruey and Woop: J. Soc. Dyers Colourisis, 39, 105 (1923). 

2 KnEcHT, Rawson, and LOwentuHa.: ‘‘A Manual of Dyeing,’ 1, 272 
(1916). 

3 Z. anorg. Chem., 64, 368 (1909); 74, 207 (1912); 84, 121 (1914). 

4Z. anorg. Chem., 74, 207 (1912). 


212 THE HYDROUS OXIDES 


ent from the others, and with few exceptions, the properties 
approached more nearly to those usually attributed to 6 oxide, 
the higher the temperature of formation. There was no apparent 
connection between the compactness of the dried powder and 
the temperature of preparation; thus, the 100° oxide contained 
the least water and was most voluminous. ‘This was possibly 
due to some variation in the conditions of drying and grinding 
of the several products, for the volume of the precipitated oxide 
depends on the temperature of formation in a perfectly regular 
fashion,! the oxide formed at the highest temperature being the 
most compact. Mecklenburg found little difference in the ease 
of peptization of his oxides with concentrated HCl, except the 
50° oxide which appeared most difficult to peptize. It seems to 
me altogether unlikely that 0 and 100° oxides should be peptized 
with equal facility. It is more probable that the difference in 
peptizibility was not detectable with concentrated acid. Meck- 
lenburg observed an increase in the precipitation concentration 
of sodium sulfate for the 100° oxide peptized by HCl on mixing 
with it the 0° oxide or one freshly prepared by hydrolysis of 
stannic chloride. From this he concludes that the different 
oxides cannot be mixtures of definite a and 8 isomers. While 
the conclusion is doubtless correct, it is certainly not justified 
from his precipitation experiments; on the contrary, these would 
seem to support the view that the oxides are mixtures. Thus, 
the so-called ‘‘sulfate value” of 100° oxide alone is 0.04 cubic 
centimeter; but when mixed with 10 to 90 per cent of a freshly 
prepared oxide it varies from 0.15 to 1.8 cubic centimeters. Since 
certain ones of his 50 and 75° oxides have sulfate values within 
the limits found for the mixtures, it might be argued that all of 
his preparations are mixtures. It should be noted in passing, 
that Mecklenburg’s observations are exactly what one should 
expect. The adsorption of the sulfate ion by the fresh oxide is 
much greater than by the 100° oxide; hence, the initial amounts 
added are all taken up by the former, and the precipitation of a 
given amount of the latter cannot take place until a higher sulfate 
concentration is reached. 

In order to determine the relative peptizability of hydrous 
stannic oxides formed at different temperatures, some experi- 


1 Weiser; J. Phys. Chem., 26, 667 (1922). 


THE HYDROUS OXIDES OF TIN AND LEAD 213 


ments were carried out! on the moist instead of the dried oxides, 
using dilute instead of concentrated acid. In these experiments 
dilute nitric acid was used, since it is known that this acid 
peptizes a oxide, whereas it has neither a peptizing nor a solvent 
action on the typical 6 oxide. Accordingly, the behavior with 
dilute nitric acid of the oxides formed under different conditions 
should give not only a measure of the relative peptizability but 
should indicate whether the oxides are definite individuals 


TaBLE XVIII.—PEpTIzATION oF Hyprous STANNIC OXIDES WITH 
Nitric ActIp 


Age of | Temperature of 


samples,| precipitation, Observations 
minutes degrees 
5 : 23 Peptized rapidly; solution cloudy after 10 min- 
utes but clear in 15 minutes’ 
40 Peptized more slowly; solution very cloudy after 


15 minutes, quite cloudy after 30 minutes but 
clear with only a slight opalescence in 45 
minutes 

58 Peptized very slowly; solution very cloudy 
after 1 hour, clearing in 2 hours, and clear 
with slight opalescence after 3 hours 


100 Peptization far from complete after 8 hours 

10 Be Peptized slowly; solution quite cloudy after 2 
hours and slightly opalescent after 4 hours 

39 Peptized very slowly; no residue, but solution 


very cloudy after 5 hours; transparent but 
cloudy after 10 hours 
58 Most peptized in 15 hours but solution opaque 
100 But little peptized 


differing in solubility, adsorbability, and peptizability, or 
whether they are mixtures of a definite a oxide peptizable by 
nitric acid with a definite B isomer not peptizable by this acid. 
One-gram samples of SnO:, were precipitated at varying tem- 
peratures, the mixtures centrifuged, and the supernatant liquid 
discarded. The precipitates were shaken with 100 cubic centi- 


1 Weiser: J. Phys. Chem., 26, 654 (1922). 


214 THE HYDROUS OXIDES 


meters of 1.25 N nitric acid either’5 or 10 minutes after precipita- 
tion, as recorded in Table XVIII. It will be noted that there is 
a distinct difference in the peptizability of the oxides prepared at 
different temperatures. The loose, finely divided and highly 
hydrous particles of the oxide formed at room temperature are 
peptized readily by nitric acid; whereas the more compact, coarser 
and less hydrous particles formed at higher temperatures are 
less readily peptized. Moreover, the 40 and 60° oxides are not 
readily peptized by dilute HNO; and so would not be designated 
as a oxides; but they are peptized after a time, which proves them 
to be neither 6 oxide nor mixtures of a and B oxides. 

Conclusions as to the relationships among the various stannic 
oxides, deduced from investigations of their behavior with chemi- 
cal reagents, are supported in a striking way by recent studies of 
their physical characteristics. Thus Pascal’ compared the 
theoretical values of the molecular magnetic susceptibilities for 
the hypothetical acids Sn(OH), and SnO(OH). with the values 
for the hydrous oxides obtained by various methods. ‘The 
results show that the ‘‘acids” are not definite compounds but 
are mixtures of anhydrous stannic oxide with water in varying 
amounts, depending on their history. Quite similar conclusions 
were reached by Yamada? from x-ray analysis of natural cassiter- 
ite and of ten samples of hydrous oxides prepared by the methods 
of (a) Zsigmondy,* (b) Schneider,* (c) Collins and Wood,® from 
SnCl, and marble (SnO.-4.2H20), (d) Graham,® (e) Rose,’ (f) 
Collins and Wood, oxidation of tin by HNOs:, (g) Engel,® (h) 
desiccating sample (f) in a vacuum, (z) drying sample (f) at 
100° (SnQz2 - 1.1H2O), (7) heating sample (f) to redness. From 
the photographs were measured the distances of the lines from 
the center, their angles, and their intensities. All the samples, 
irrespective of their history, contained a similar central nucleus; 
hence, the physical difference among them is due not to chemi- 

1 Compt. rend., 175, 1063 (1922). 

2 J. Chem. Soc. Japan, 44, 210 (1923); Cf. Posnsax: J. Phys. Chem., 30, 
1073 (1926). 

3 Tnebig’s Ann. Chem., 301, 361 (1898). 

4Z. anorg. Chem., 5, 82 (1894). 

5 J. Chem. Soc., 121, 441 (1922). 

6 Pogg. Ann., 123, 538 (1864). 

7 Pogg. Ann., 75, 1 (1848). 

8 Ann. chim. phys., (3) 12, 463 (1844), 


THE HYDROUS OXIDES OF TIN AND LEAD 215 


eal differences but to the physical structure and to the manner in 
which water adheres to the surface of the oxide granules. 


STANNIC OXIDE SOLS 


Formation.—Colloidal stannic oxide almost free from electro- 
lytes was first prepared by Graham! by adding alkali to stannic 
chloride solution or hydrochloric acid to sodium stannate solu- 
tion short of precipitation and dialyzing the resulting solutions. 
In both cases a gel was first formed on the dialyzer, but this went 
into colloidal solution again as the purification was continued. 
The sol was negatively charged, doubtless owing to the presence 
of a small amount of free alkali. Excess of the latter was 
removed by the addition of a few drops of tincture of iodine. As 
noted previously, Graham was able to boil the colloid without 
precipitating it, thereby forming colloidal 6 oxide. - His prepa- 
‘rations were fairly pure and so were readily coagulated by salts 
and acids. 

Schneider? dialyzed the sol formed by adding ammonia to 
stannic chloride short of precipitation; and Zsigmondy? peptized 
with ammonia the thoroughly washed oxide formed by hydrolysis 
of a dilute solution of stannic chloride. The amount of ammonia 
required was very small; in one experiment, a single drop contain- 
ing approximately 0.03 gram of ammonia sufficed for the peptiza- 
tion of 1.45 grams of oxide. Any excess ammonia was removed 
by heating the colloid to boiling, thus doing away with the neces- 
sity for dialysis. Sols prepared in this way were negatively . 
charged and were readily precipitated by electrolytes, particu- 
larly those having strongly adsorbed cations. ‘The properties 
of the precipitated oxides lay between those of the typical a and 
6 oxides, and Zsigmondy believed them to be mixtures of the two 
forms, the usual properties of each being modified by the presence 
of the other. This view is probably incorrect. 

As previously noted, hydrous stannic oxide freshly prepared 
at room temperature, is readily peptized by dilute mineral acids; 
while the aged oxide is peptized by concentrated HCl and H2SO, 
under suitable conditions, but not by HNO;. The sols are posi- 

1 Phil. Trans., 121, 213 (1861). 


2Z. anorg. Chem., 5, 82 (1894). 
3 Liebig’s Ann. Chem., 301, 361 (1898). 


216 THE HYDROUS OXIDES 


tively charged owing to preferential adsorption of hydrogen 
ion,! as evidenced by the low precipitation value of sulfate 
ion in the presence of considerable excess of hydrogen ion.? 
Biltz® obtained a fairly pure positive sol by dialysis of stannic 
nitrate prepared by metathesis of stannic chloride and lead 
nitrate. Metallic tin melted in an electric arc furnace and blown 
with air gives very finely divided SnO, which can be peptized 
by 0.02 to 0.01 N hydrochloric acid. 

Ageing.—Attention has been called to the transformation of 
a oxide peptized by dilute HClinto the 8form. This transforma- 
tion has been followed in a number of ways, a few of which will 
be mentioned: Léwenthal®> found that potassium ferrocyanide 
could be removed from solution completely by the addition of a 
dilute solution of stannic chloride, but that the older the tin 
solution, the more was necessary to precipitate a definite amount 
of ferrocyanide and the greater was the relative amount of tin 
in the precipitate. This is shown in Table XIX. L6éwenthal’s 
observations were confirmed by Lorenz,® who assumed that the 
ferrocyanide was removed as SnFe(CN)., and that more old 
stannic chloride solution was necessary on account of the lower 


TaBLE XIX 


C ition of ipi- 
Amount of SnCl, solu- omposition OF precip 


tate 
Age of SnCl, solution, | tion to precipitate 0.5 Mole ore j H 

days gram K4Fe(CN)6., cubic Oe eae 
centimeters mols of K,Fe(CN)¢ 

0 6 1.5 

7 105 2.3 

21 27 6.5 

126 32 7.5 


1Cf. ZocHER: Z. anorg. Chem., 112, 46 (1920). 

2 LOWENTHAL: J. prakt. Chem., 56, 366 (1852); MmcKLENBURG: Z. anorg. 
Chem., 74, 207 (1912). 

3 Ber., 35, 443 (1902). 

4 GoLpscHMipT and KonuscuHtrrer: British Patent 189706 (1922). 

6 J. prakt. Chem., TT, 321 (1859). 

6Z. anorg. Chem., 9, 369 (1895), 


THE HYDROUS OXIDES OF TIN AND LEAD 217 


concentration of stannic ion resulting from slow hydrolysis. This 
explanation is unsatisfactory! for two reasons: first, because the 
hydrolysis of inorganic salts takes place much more rapidly than 
Lorenz assumed; and, second, because the composition of the 
precipitate is not SnFe(CN). but is variable, containing more and 
more tin the older the solution. The true explanation of Léwen- 
thal’s observations les in the ageing of the colloidal hydrous 
oxide. ‘The addition of K,Fe(CN). causes coagulation of the 
colloid. Since the particles of a newly formed colloid are smaller 
and have a greater adsorption capacity than those of an older 
colloid, less of the former is necessary to adsorb completely a 
given amount of ferrocyanide and the ratio of tin to ferrocyanide 
in the precipitate is relatively low. As the colloid ages, it 
becomes less stable; and the adsorption capacity falls off so that 
more colloid is necessary to adsorb a definite amount of ferro- 
cyanide, and the ratio of tin to ferrocyanide becomes quite 
large.? 

Tartaric acid was found by Lowenthal to prevent the ageing of 
colloidal hydrous stannic oxide. While this may be due to some 
specific action of tartrate ion, I am inclined to attribute it to 
the formation of a definite complex, obtainable in crystalline form 
if desired.’ 

The age of colloidal stannic oxide may be determined roughly 
by treating with stannous chloride. The colloid prepared from 
newly formed stannic oxide is not precipitated by stannous 
chloride,t whereas the aged colloid is thrown down as a yellow 
precipitate by this reagent. The precipitate is variable in com- 
position,® consisting of hydrous stannic oxide that has adsorbed 
varying amounts of stannous chloride under the different condi- 
tions of precipitation. Collins and Wood observed a small 


1 MECKLENBURG: Z. anorg. Chem., 65, 372 (1909). 

2 Cf. Barrorep: J. prakt. Chem., 101, 368 (1867). 

3 ROSENHEIM and Aron: Z. anorg. Chem., 39, 170 (1904). 

4LOwENTHAL: J. prakt. Chem., T7, 321 (1859); Brron: J. Russ. Phys.- 
Chem. Soc., 37, 933 (1905). 

5 Fremy: Ann. chim. phys., (3) 12, 462 (1844); 23, 393 (1848); Scuirr: 
Liebig’s Ann. Chem., 120, 47 (1861); TscuermMakK: J. prakt. Chem., 86, 334 
(1862). 

6 Weiser: J. Phys. Chem., 26, 674 (1922); Cotuins and Woop: J. Chem. 
Soc., 123, 452 (1923). 


218 THE HYDROUS OXIDES 


increase in adsorption with increasing 6 character of the 
hydrous oxide, indicating that some factor other than size of 
grain is involved. As is usual, stannic oxide shows a stronger 
_tendency to adsorb tin ions than chloride ions. On account of 
the usual strong adsorption of hydrogen ion, the adsorption of 
stannous chloride is somewhat less in the presence of hydrochloric 
acid.! 

It may be mentioned in passing that hydrogen sulfide precipi- 
tates stannous sulfide from a colloidal solution of the fresh 
oxide in dilute hydrochloric acid; whereas, from the aged col- 
loid, hydrogen sulfide precipitates hydrous stannic oxide that is 
converted only very slowly into stannous sulfide. The explana- 
tion of this behavior is evident when we consider the difference 
in solubility of the new and old oxide. In the new colloid pre- 
pared by peptization with hydrochloric acid there is some stannic 
ion, the removal of which by precipitation as stannous sulfide 
results in further solution and subsequent precipitation until all 
is thrown down as sulfide; while the aged colloid contains but a 
negligible amount of stannic ion and the precipitate with hydro- 
gen sulfide is almost entirely the hydrous oxide. 

It thus appears that we may have colloidal solutions of any 
number of hydrous stannic oxides, each differing from the others 
in the size of the primary hydrous particles and, hence, in their 
reactivity, adsorbability, and stability under given conditions. 
As a rule, the particles tend to agglomerate into denser and less 
reactive secondary aggregates on standing, but the reverse process 
goes on in the presence of fairly concentrated hydrochloric acid or 
alkali. As with the precipitated oxide, there is no ground for 
assuming that the different colloidal solutions are mixtures of 
colloidal a with colloidal 6 particles in varying proportions. 

Behavior with Colloidal Metals.—One of the most characteris- 
tic properties of colloidal hydrous stannic oxide is its protective 
action on colloidal metals. It is well known that a gold solution 
treated with stannous chloride first gives a red coloration fol- 
lowed by the settling out of a purple or brown precipitate known 
as gold purple of Cassius from its discoverer, Andreas Cassius, 

1Cf., however, CoLLins and Woop: J. Chem. Soc., 123, 452 (1923). 


* JORGENSEN: Z. anorg. Chem., 28, 140 (1901); Barone J. praki. aer 
101, 368 (1867). 


THE HYDROUS OXIDES OF TIN AND LEAD 219 


of Leyden. Because of its wide use as a pigment in the ceramic 
industry, a number of recipes have been given for its preparation. 
The substance varies in color and composition with the method 
of formation. Certain earlier investigators, as Richter and Gay 
Lussac, believed purple of Cassius to be a mixture; but Berzelius 
thought it must be a definite compound. The latter view is 
supported by several facts: Purple of Cassius is purple in color, 
while a mixture of gold and stannic oxide is brick red; gold is 
not separated from purple of Cassius with aqua regia, whereas it 
is from a mixture; mercury does not extract gold from the purple 
as it does from a mixture; and finally, the freshly prepared purple 
is dissolved by ammonia, forming a purple liquid. In spite of 
this evidence we now know that Berzelius’ view is incorrect. 
Debray' believed that gold forms a kind of color Jake with stannic 
oxide, which is soluble in ammonia. Schneider? emphasized 
the colloidal character of the purple and concludes rightly that its 
ammoniacal solution is a mixture of colloidal gold with colloidal 
hydrous stannic oxide. Supporting Schneider’s view, Zsigmondy* 
showed that a mere trace of ammonia will dissolve a large amount 
of freshly precipitated purple and that this purple solution will 
not pass through parchment during electrolysis, as electrolytes 
do. He settled the question once for all by precipitating with 
nitric acid suitable mixtures of colloidal gold with colloidal stan- 
nic oxide, obtaining purples almost identical with those prepared 
in other ways. The gold does not combine chemically with 
stannic oxide, but the usual properties of the former are masked 
by the protective action of the latter. 

Colloidal gold was found by Miiller+ to impart a red coloration 
to a number of substances® and Moissan® obtained purples by 
distilling gold with tin, alumina, magnesia, zirconia, silica, 
and lime. Substances similar to gold purples have been prepared 
with other metals. Thus Wohler’? obtained silver purples 
similar to the gold pigment by mixing silver nitrate with stannous © 

1 Compt. rend., 75, 1025 (1872). 

2Z. anorg. Chem., 5, 80 (1894). 

3 Liebig’s Ann. Chem., 301, 361 (1898). 

4 J. prakt. Chem., (2) 30, 252 (1884). 

5 ANTony and Luccusst: Gazz. chim. ival., (2) 26, 195 (1896). 


6 Compt. rend., 141, 977 (1905). 
7 Kolloid-Z., T, 248 (1910). 


220 THE HYDROUS OXIDES 


nitrate; and Lottermoser' prepared the former synthetically in 
the same manner as Zsigmondy prepared the latter. Wohler? 
has also made an analogous platinum combination. All of these 
so-called ‘‘purples’” are colloidal in nature, the composition 
varying with the conditions of formation. Their colloid chemis- 
try is chiefly that of the hydrous oxide. When freshly prepared, 
the purples are readily peptized by ammonia or dilute hydro- 
chloric acid; but when dried, there is little or no peptizing action 
even by concentrated ammonia or hydrochloric acid. 

Behavior with Other Hydrous Oxides.—In analytical chemis- 
try, the usual method of estimating tin consists in oxidizing it to 
insoluble stannic oxide and weighing it as such. Itis well known, 
however, that the oxide formed in this way is always contam- 
inated by other substances present in the solution, such as iron, 
bismuth, copper, and lead. Rose*® observed that when iron is 
present in small amounts, the stannic oxide precipitated from 
nitric acid solution is contaminated by it; but that when any 
considerable quantity of iron is present, both the iron and tin 
remain in solution. Lepez and Storch‘ digested tin with nitric 
acid containing iron, and obtained solutions of variable stability 
depending on the relative amounts of the two metals present; 
solutions containing 2 atoms or less of tin to 1 of iron could be 
boiled and even evaporated to dryness in a vacuum. Concen- 
trated nitric acid threw out of the solutions a yellowish precipi- 
tate that redissolved on dilution; sulfuric acid. and sulfates caused 
a permanent precipitate; while acetic acid and alkali chlorides 
and nitrates caused no precipitation. By evaporating different 
solutions, the authors claimed to get compounds having such 
formulas as 1.8Sn0;2 ¥ HO : Fe.O3 ° 1.8N.0; and 4SnO0, : H.O = Feo- 
QO3:1.1N20;. When a mixture of hydrous ferric and stannic 
oxides was thrown down from the mixed nitrates by a slight 
excess of ammonia and the precipitate washed free from ammo- 
‘nium nitrate, this precipitate, still containing a trace of ammonia, 
dissolved in water to a clear solution. Removal of ammonia by 
dialysis resulted in precipitation; but the addition of a trace of 


1“ Anorganische Kolloide,’”’ 53 (1901). 

2 Kolloid-Z., 2nd Supplement, III (1907). 
3 Rose: Pogg. Ann., 112, 164 (1861). 

4 Monaishefte, 10, 283 (1889). 


THE HYDROUS OXIDES OF TIN AND LEAD 221 


ammonia again caused complete solution. Chromic nitrate 
behaved like ferric nitrate; but aluminum, uranium, cobalt, 
nickel, and copper nitrates did not cause solution of stannic 
oxide. 

The phenomena described by Lepez and Storch strongly 
suggest that their ferric-stannic mixtures were not salt solutions 
but were either colloidal solutions of hydrous ferric oxide pep- 
tized by hydrous stannic oxide or colloidal hydrous stannic 
oxide peptized by ferric and hydrogen ions. ‘The real nature of 
the mixtures was shown by two series of experiments.! In the 
first experiments, mixtures of freshly prepared oxides were 
treated with 100 cubic centimeters of 0.01 N ammonium hydrox- 
ide as shown in Table XX. The results are quite conclusive: 
-Hydrous stannic oxide is peptized by hydroxyl ion, while hydrous 
ferric oxide is not. However, the colloidal stannic oxide adsorbs 
ferric oxide and carries it into colloidal solution as long as tin 
is present in excess. At the same time, hydrous ferric oxide 
adsorbs stannic oxide and tends to take it out of colloidal solu- 
tion so that no tin remains peptized when the former is present 
in large excess. This is quite analogous to the behavior of 


TABLE XX.—PEPTIZATION OF MrIxTuURES oF HypRovUS STANNIC OXIDE AND 
Hyprovus Ferric OxipE witH 0.01 N AmMMontum HypROXIDE 





Mixed oxides prepared 
from ; 
N SnCl, + N FeCl;, Observations 
cubic centimeters 


9.5 0.5 Clear colorless colloidal solution 

9.0 1.0 Clear colloidal solution with yellow tinge 

8.5 He Clear yellow colloidal solution 

8.0 20 Clear yellow colloidal solution 

7.5 2.5 Clear reddish-yellow colloidal solution 

6.0 4.0 But little ferric oxide peptized; supernatant liquid 
cloudy 

5.0 5.0 No ferric oxide peptized; supernatant liquid clear 
and colorless 

2.0 ao. No peptization of either hydrous oxide 


1 WerIsER: J. Phys. Chem., 26, 678 (1922). 


222 THE HYDROUS OXIDES 


hydrous chromic oxide with the hydrous oxides of iron, manga- 
nese, cobalt, nickel, copper, and magnesium.! On account of the 
mutual adsorption of hydrous stannic oxide and hydrous ferric 
oxide, we should expect the precipitate obtained by mixing 
positive ferric oxide sol with negative stannic oxide sol to contain 
appreciable amounts of both oxides. It would be interesting 
to know whether the precipitate obtained by mixing yellow 
colloidal ferric oxide? with colloidal stannic oxide under suitable 
conditions can be ignited without becoming red.* 

In a second series of experiments, freshly prepared samples of 
hydrous stannic oxide were treated with mixtures of ferric nitrate 
and nitric acid, as shown in Table XXI. The explanation of the 
observations is fairly simple. As previously noted, hydrous 
stannic oxide peptized by nitric acid coagulates spontaneously, * 
since the aged oxide is neither peptized nor dissolved by this acid. 
Ferric nitrate peptizes this oxide both when newly formed and 
when aged. Accordingly, if freshly prepared hydrous stannic 
oxide is peptized either by ferric nitrate or by a suitable mixture 


TaBLE XXI.—CoLiompaAL STANNIC OXIDE PEpTIZED BY FE(NO3)3 AND 








HNO; 
0.38 gram SnO; + xH.0 peptized Observations 
by 
N HNO, + N Fe(NOs)3 + H.O, 
cubic centimeters After 1 day After 1 week 
40 0 10 All precipitated 
39 1 10 Very cloudy; part- | All precipitated 


ly precipitated 


38 2 10 Slightly cloudy Very cloudy 
37 3 10 Slightly opalescent) Slightly cloudy 
36 4 10 Clear Slightly opalescent 
35 5 10 Clear Slightly opalescent 
34 6 10 Clear Clear 

0 40 10 Cloudy Clear 


1 NorTHCOTE and Cuurcu: J. Chem. Soc., 6, 54 (1854); Naas: J. Phys. 
Chem., 19, 331 (1915). 

a Wanreuh: J. Phys. Chem., 24, 322 (1920). 

3 KEANE: J. Phys. Chemin: 734 (1916); Scoretz: Ibid., 21, 570 (1917); 
Yor: [bid., 25, 196 (1921). 


THE HYDROUS OXIDES OF TIN AND LEAD 223 


of ferric nitrate and nitric acid, coagulation does not take place 
on standing or boiling, on account of the stabilizing action of the 
strongly adsorbed ferric ion; but if the concentration of ferric 
ion in the nitric acid solution is too low, partial coagulation takes 
place as shown in the table. ! 
Stannic Oxide Jellies—When a colloidal solution of hydrous 
stannic oxide is evaporated, a transparent jelly is obtained; 
while precipitation with electrolytes is said always to give a 
gelatinous precipitate and not a jelly. Since hydrous stannic 
oxide apparently possesses the desired properties, one should 
expect to get stannic oxide jellies by precipitation from colloidal 
solution under suitable conditions. This conclusion was con- 


TaBLeE X XII.—PRECIPITATION OF COLLOIDAL STANNIC OXIDE BY 


ELECTROLYTES 
Electrolyte 
Amount Concentration, Observations 
Formula | added, cubic | milliequivalents 
centimeters per liter 
BaCl, 3.00 N/100 3.00 Clear transparent jelly 
BaCl, | 3.50 N/100 3.50 Clear transparent jelly; very 
firm 
BaCl, 3.75 N/100 Shaves Jelly somewhat cloudy and 
slightly synerized 
BaCl, 4.25 N/100 4.25. Gelatinous precipitate 
SrCl, 3.50 N/100 3.50 Clear; somewhat viscous 
SrCl, 4.00 N/100 4.00 Clear transparent jelly 
SrCl, 4.50 N/100 4.50 Clear transparent jelly; very 
‘y firm | 
SrCl, 5.00 N/100 5.00 Cloudy jelly; synerized slightly 
NaCl 2.00 N/10 20.00 Clear; viscous 
NaCl 2.20 IV 710 22.50 Soft cloudy jelly 
NaCl 2.50 N/10 25 .00 Soft cloudy jelly 
NaCl 2.75 N/10 27.50 Gelatinous precipitate 
HCl 1.50 N/50 3.00 Clear 
HCl 1.75 N/50 3.50 Clear transparent jelly 
HCl 2.00 N/50 4.00 Clear transparent jelly 
HCl 2.25 N/50 4.50 Cloudy jelly; synerized slightly 
HCl 2.50 N/50 5.00 Gelatinous precipitate 





1 Zsiamonpy: ‘“‘Chemistry of Colloids,’ translated by Spear, 155 (1917). 


224 THE HYDROUS OXIDES 


firmed! by precipitating a sol prepared by Zsigmondy’s method 
containing 28 grams SnO, per liter. The results as recorded in 
Table X XII are in accord with the general theory.” The jellies 
formed under the most favorable conditions were very firm and 
stable, remaining unbroken after standing several months. 
If just the right amount of electrolyte is added, the jelly may be 
converted into a sol by shaking and on standing will again set to 
a jelly.’ 

As might be expected, jellies are not formed by adding an excess 
of alkali to a stannic salt and allowing the sol to stand, the reason 
being that alkalies have a slight solvent action as well as a pep- 
tizing action on the hydrous oxide. This solvent action causes 
the precipitate which comes out spontaneously to consist of 
large granular particles instead of the fine chains or filaments 
that make up a jelly structure. | 


Hyprous STANNOUS OXIDE 


Hydrous stannous oxide is precipitated as a yellow highly 
gelatinous mass by adding alkali hydroxide or carbonate to a 
solution of stannous chloride. Ditte* assigned the formula 
38nO:2H.O to the compound precipitated with alkali and dried 
at 110°; and Schaffner® claimed to get 25nO-H2O by precipitat- 
ing with carbonate and drying below 80°. Bury and Partington® 
prepared five different samples, using ammonia, carbonate, and 
alkali as precipitants, both in the air and in an atmosphere of 
carbon dioxide. With alkali and carbonate, the samples pos- 
sessed a yellow tinge from the start; but with ammonia, they 
were white when first prepared, becoming yellow on drying. 
After drying over phosphorus pentoxide, the samples gave an 
analysis for tin corresponding to the compound 3Sn0:2H,0; 
but the water content varied from 7.11 to 8.82 per cent, the cal- 
culated value for the compound being 8.16 per cent. These data 
are insufficient to establish the identity of the alleged hydrate. — 


1 Weiser: J. Phys. Chem., 26, 681 (1922). 

2° Chapa; ps 26: 

3 ScHALEK and SzEavaRi: Kolloid-Z., 33, 326 (1923). 
4 Ann. chim. phys., (5) 27, 145 (1882). 

6 Tniebig’s Ann. Chem., 51, 168 (1844). 

6 J. Chem, Soc., 121, 1998 (1922), 


THE HYDROUS OXIDES OF TIN AND LEAD 225 


The gelatinous oxide is very difficult to wash free from the 
mother liquor, and if the washing is carried too far, the oxide 
passes through the filter, forming a sol. Bury and Partington 
observed the slow transformation of the hydrous oxide sol into a 
crystalline oxide, a part of which precipitated on the walls of the 
flask and a part remained in suspension, giving a creamy-yellow 
liquid that glistened on shaking. 

The hydrous oxide loses water even in contact with water, 
going over to the anhydrous state. In the presence of alkali, 
the rate of dehydration is acclerated and the precipitate darkens. 
The effect of a trace of alkali is evidenced by the darkening of 
samples of hydrous oxide in contact with the walls of glass vessels 
and the absence of darkening in samples stored in quartz vessels. ! 
The color of the anhydrous oxide varies from dark gray to black, 
depending on the method of preparation. It is not likely that 
the different shades represent different modifications. Roth? 
claims to get a red crystalline oxide by the action of an acetic acid 
solution of SnO, on the gelatinous oxide, but Bury and Partington 
were unable to confirm this result. 

Hydrous stannic oxide is dissolved by alkali forming NaHSnOg, 
unless the alkali is quite concentrated when Na2SnOsz is obtained.* 
Schneider‘ reports the preparation of a colloidal solution of Sn2.O3 
by adding a very dilute solution of stannous chloride to a sol of 
hydrous stannic oxide and dialyzing in the absence of air. The 
sol is a yellow clear neutral liquid with strong reducing properties; 
thus, the addition of gold chloride gives gold purple. In all 
probability, the sol is not Sn2O3 but a mixture of hydrous stannic 
oxide and basic stannous chloride® stabilized by the hydrous 
oxide. 


Hyprovus LEAD MONOXIDE 


It is somewhat surprising not to find a reference to an analysis 
of precipitated lead monoxide which corresponds to the formula 


1 Bury and Partineton: J. Chem. Soc., 121, 1998 (1922). 

2 Abege’s “Handbuch anorg. Chem.,” 4, IJ, 573 (1909). 

3 Hantzscu: Z. anorg. Chem., 30, 289 (1902); GoLtpscumipT and EcKarpr: 
Z. physik. Chem., 56, 385 (1906); KorticHEen: [bid., 33, 129 (1900). 

47Z. anorg. Chem., 5, 83 (1894). 

5 Carson: J. Am. Chem. Soc., 41, 1969 (1919). 


226 ‘ THE HYDROUS OXIDES 


Pb(OH)s. However, it is tacitly assumed by Ditte'! and by 
Wood? that the compound freshly precipitated from lead nitrate 
solution by alkali and ammonia is lead hydroxide, when, as a 
matter of fact, the mass is either a basic salt’ or contains adsorbed 
alkali, unless it is digested repeatedly with sodium hydroxide 
solution which converts it to pure hydrous oxide.’ Hydrates 
having the formula 2PbO - H.O4 and 3PbO - H2O® have been 
reported; but Glasstone® repeated the experiments of various 
authors and failed to get any product which could be described 
as either of these compounds. The water content of the oxide 
precipitated from lead acetate solution with alkali was lower 
the greater the dilution, and the higher the temperature of the ~ 
reacting solutions. A variety of hydrous products were heated 
in a current of air at 105 to 110° until decomposition set in, as 
evidenced by a slight change in color, and the water content was 
determined. In every case the oxides contained 3.08 to 3.13 
per cent of water, corresponding approximately to the com- 
position 5PbO:2H.O. From this, Glasstone assumes that every 
form of the oxide, whether crystalline or amorphous, is the same 
chemical entity with varying amounts of adsorbed water; but 
whether the substance is a single hydrate or a solid solution of 
two hydrates is left undecided. Glasstone rules out the possi- 
bility of the precipitated oxide being hydrous PbO, since at 105° 
there is always a color change when the water content is reduced 
to approximately 3.18 per cent. It would seem that the cause 
and nature of this color change should be investigated further. 
If the temperature were higher, it might be ascribed to the 
transformation of yellow to red oxide or to the formation of 
minium. Glasstone quotes Winkelblech’ as reporting a loss of 


1 Compt. rend., 94, 1310 (1882). 

2 Woon: J. Chem. Soc., 97, 878 (1910). 

3 WINKELBLECH: Liebig’s Ann. Chem., 21, 21 (1837). 

4ScHAFFNER: Liebig’s Ann. Chem., 51, 175 (1844); Ltprexine: Am. Chem. 
J., 18, 120 (1891); Ocata and Karun: J. Pharm. Soc. Japan, 492, 75 (1923). 

5 PavEn: Ann. chim. phys., (4) 8, 302 (1866); Mutprr: Dammer’s ‘‘ Hand- 
buch anorg. Chem.,”’ 2, II, 524; PLetissnreR and AUERBACH: Abegg’s ‘‘ Hand- 
buch anorg. Chem.,’’ 3, II, 677 (1909); cf. also BOrreER: Z. physik. Chem., 
46, 580 (1903); Lorenz: [bid., 12, 436 (1897). 

6 J. Chem. Soc., 121, 58 (1922). 

7 Liebig’s Ann, Chem., 21, 25 (1837). 


THE HYDROUS OXIDES OF TIN AND LEAD 227 


0.6 per cent of water at 105° before decomposition started; 
whereas what Winkelblech actually observed was a decomposition 
of basic nitrate on heating to some unrecorded temperature, 
red fumes being evolved and minium being formed.! 

Lead monoxide occurs in nature in two crystalline forms, 
litharge and massicot, which are yellow and red, respectively. 
Hydrous lead oxide precipitated in the cold is white, but heating 
with 10 per cent alkali converts it to yellow or yellowish-green 
oxide. If the precipitation is carried out in boiling alkali solu- 
tion, red anhydrous oxide is obtained.’ The relationships among 
the various forms worked out by Ruer? may be represented as 
shown in Table XXIII.* 













TaBLeE XXIII 
alkali 
Reddish brown 5% NaOH lead acetate-———————>white hydrous 
Heat to 620° — — solution oxide 
and cool => 
50 se 2% KOH 
~ lhe 7 5% KOH boiling 
’ Lox alkalis 
heat to 720° and coo] ms 1 ‘| 
ee red 50 % alkali hot yellow or 
eg re a tt, a —— > yellowish 
50 % alkali ho po 


Treen 
‘3 ‘ 


| 
Yellow —> 
1 








Commercial 
reddish brown 


The difference between the two forms of the oxide is commonly 
attributed to polymorphism, the red being regarded as the more 
stable form at the ordinary temperature and at all temperatures 
up to a transition point that has not been determined.* This 
view is supported by the observed differences in crystal structure,° 
density, and solubility® of the two forms. The yellow crystals 


1 Cf. Burton: Dinglers polytech. J., 167, 361 (1863). 

2 Ruger: Z. anorg. Chem., 50, 265 (1906). 

3 GLASSTONE: J. Chem. Soc., 119, 1689 (1921). 

4 JamceR and Germs [Z. anorg. Chem., 119, 147 (1922)] give 587°. 

6’ NORDENSKIOLD: Pogg. Ann., 114, 619 (1861); Larsen: U. S. Geol. 
Survey Bull., 679, 105 (1921). 

6 GruTHER: Liebig’s Ann. Chem., 219, 56 (1883); RuER: Z. anorg. Chem., 
50, 265 (1906). 


228 THE HYDROUS OXIDES 


are rhombic, biaxial, and positive in action on polarized light, 
while the red are tetragonal, uniaxial, and negative in action on 
polarized light. | 

Recently, however, Glasstone suggests a closer relationship 
between the two forms than that of allotropy. He calls atten- 
tion to the transformation of all red forms to brownish yellow 
on grinding and all yellow forms to red on heating with con- 
centrated alkali. These observations indicate a close connection 
between color and size of particles, the large particles appearing 
red and the small ones yellow. In further support of this view, 
Glasstone! made solubility determinations both gravimetric and 
electrometric, on eight different preparations varying in color 
from lemon yellow through reddish brown to red and found 
approximately the same value, irrespective of the color. Micro- 
scopic examinations likewise indicate that the red forms are made 
up of larger particles, the yellow samples being agglomerates of 
small particles which are almost identical with the finely divided 
red forms. ‘These data are misleading, however, as Appleby 
and Reid? succeeded in making well-defined crystals of the two 
forms which differ not only in crystal structure but in solubility, 
the yellow being 1.8 times as soluble as the red. Glasstone’s 
products were mixtures, and the constant solubility he observed 
was the value for the more soluble yellow form. Appleby and 
Reed’s conclusions were confirmed by Kohlschiitter and Scherrer* 
who examined the two forms with x-rays and found them struc- 
turally different. There is, therefore, no doubt of the poly- 
morphism of lead. oxide; but it is not improbable that each 
crystalline form should show variations in color between yellow 
and red by varying the size of the particles. Indeed, this is 
what we find with the two crystalline forms of mercuric oxide;* 
and it is known that grinding red lead oxide crystals changes 
them to brownish yellow, probably without changing the crystal 
structure. 


1 J. Chem. Soc., 119, 1689, 1914 (1921). 

2J. Chem. Soc., 121, 2129 (1922). 

° Helvetica chim. Acta, 7, 387 (1924); cf. KonLtscHiiTTER and Rosgstt: 
Ber., 56, 275 (1923). 

4 Page 173, 


THE HYDROUS OXIDES OF TIN AND LEAD 229 


Lead oxide dissolves slightly in dilute alkali but appreciably in 
concentrated alkali, forming plumbite.? The alkali plumbites 
are strongly adsorbed by cotton. By washing the fiber thor- 
oughly, the salts are hydrolyzed, giving alkali that dissolves out 
and lead oxide that is retained by the fiber and acts as a mordant.* 

Although the compound Pb(OH), is not known, it is an inter- 
esting fact that boiling lead sulfate or chloride with aqueous 


Milligrams of Lead in!00 ce. 





PbO 80 60 A0 20 0 
0 20 40 60 80  —- PbC0, 


Molecular Percentages in Solid Phases 


Fria. 14.—Composition of white lead. 


sodium carbonate gives Pb(OH).- 2PbCOs, basic lead carbonate 
or white lead. The same compound is formed by bringing - 
anhydrous PbO and lead carbonate together in a sodium acetate 
solution. In Fig. 14 are given the results of allowing various 
mixtures of PbO and PbCO; to stand in contact with 20 per cent 
sodium acetate solution at 75° for 12 hours and subsequently 
analyzing both solutions and precipitates. ‘The diagram shows 

1 Bert and AusTERWEIL: Z. Elektrochem., 13, 165 (1907). 

2 HantzscH: Z. anorg. Chem., 30, 305 (1902); Herz and Fiscusr: [bid., 
$1, 454 (1902). 

3 BONNET: Compt. rend., 117, 518 (1893). 

4 SALVADORI: Gazz. chim. ital., 24, I, 87 (1904). 

6 Hawuey: J. Phys. Chem., 10, 654 (1906). 


230 THE HYDROUS OXIDES 


that PbO and PbCO; do not form solid solutions but a compound 
containing 1 mol of the former to 2 of the latter. Water deter- 
minations show the formula to be Pb(OH).2: 2PbCQ3.! 

Hydrous plumbic oxide and thorium oxide mutually adsorb 
each other. The former, suspended in water, is carried into 
colloidal solution by thorium acetate which hydrolyzes to give 
colloidal thorium oxide; but if an excess of hydrous lead oxide is 
shaken with a thorium acetate solution, thorium oxide is carried 
down along with the lead.’ 


Hyprous LEAD PEROXIDE 


Electrolysis of a weak alkaline solution of lead sodium tartrate 
gives a black lustrous compound reported to be PbO2: H20.* 
The hydrate is also said to form during the electrolysis of a sodium 
chloride solution in which litharge is suspended.* Electrolysis 
of acid or neutral solutions of lead salts usually gives the anhy- 
drous oxide;®> but Wernicke reports the formation of lead peroxide 
with variable quantities of water by electrolyzing dilute solutions 
of lead nitrate for varying lengths of time. It is probable, there- 
fore, that the so-called monohydrate or metaplumbiec acid is 
really a hydrous oxide. ‘There are well-defined metaplumbates 
as well as orthoplumbates, however, the latter derived from the 
hypothetical acid H4PbO, or PbO2.: 2H20. According to Bellucci 
and Parravano® the metaplumbates such as K2PbO3-3H2O 
should be regarded as salts of an acid HePb(OH)., both be- 
cause of isomerism with the corresponding potassium stannate 
and platinate and because they cannot be dehydrated without 
decomposition. 

Alkali metaplumbates hydrolyze strongly in water,’ giving 
colloidal hydrous lead peroxide® together with potassium 


1Cf. PueissNER and AvERBACH: Abegg’s ‘‘Handbuch anorg. Chem.,”’ 
4, II, 726 (1909). 

2 $ziLaRD: J. chim. phys., 5, 645 (1907). 

3 WERNICKE: Pogg. Ann., 141, 109 (1870). 

4Chemische Fabrik. Griesheim-Elektron, German Patent 124512; Chem. 
Zenir., II, 1101 (1901). 

5 W6HLER: Liebig’s Ann. Chem., 90, 383 (1854); GruruER: Ibid., 96, 382 
(1865); FEHRMANN: Ber., 15, 1882 (1882). 

6 Z. anorg. Chem., 60, 107 (1906); Atti accad. Lincei, 14, I, 378, 457 (1905). 

7 PARRAVANO and Caucaanti: Gazz. chim. ital., 37, II, 264 (1907). 

8 BELLUCCI and PARRAVANO: Atti accad. Lincet, 15, II, 542, 631 (1906). 


THE HYDROUS OXIDES OF TIN AND LEAD 231 


hydroxide, a great part of which can be dialyzed out without 
precipitation taking place; but if the dialysis is carried too far, 
the hydrous oxide gelatinizes on the dialyzer. A sol containing 
0.32 gram PbO: and 0.008 gram K;O, that is, 175 mols of per- 
oxide to 1 of alkali, is neutral in reaction and gives no depression 
of the freezing point of water. It possesses a chestnut-brown 
color, and is perfectly clear in transmitted light but cloudy by 
reflected light; it can be diluted, heated to boiling, frozen or 
evaporated on the water bath without coagulation. By evapor- 
ating off the excess water, a jelly is formed which can be repep- 
tized by water; drying the jelly renders it non-peptizable. The 
sol is negatively charged and is quite sensitive to the action of 
certain electrolytes, particularly those having multivalent 
cations. The order of precipitating power of chlorides, begin- 
ning with the greatest, is: Fe’, Al’, Ca’, Sr’, Ba’, Mg”, 
oeeiie vin | Ni’, Co’, Cu’, NHi, Cs’, li, K’, Rb, Na. 
The effect of stabilizing ions having the same charge as the sol is 
quite marked. Thus, potassium salts of AsO4’”’, CO3”, C204” 
and IO,’ do not coagulate the sol; FeCNe’’””, ClO’, ClO3’, MnO,’ 
Cr,0,7", Br Oe CNS’, SO4”, NO3’, 10,3’, C.H;30,’ precipitate it 
partially; and F’, C4H,0,"’, Fe(CN).’’", and I’ precipitate it 
completely. The slight stabilizing action of ferrocyanide and 
tartrate ions is anomalous and should be reinvestigated. 

If an alkali solution of PbOz is treated with potassium or cal- 
cium plumbate, an amorphous orange-yellow powder is obtained 
which analyzes approximately for Pb.O3;-2H,O! after drying 
over sulfuric acid. It loses only about one-third of its water at 
170° but at higher temperatures it can be dried completely, 
apparently without decomposition. The sesquioxide can be 
broken up by acids into PbO and PbO, and is regarded as a 
compound of the two, PbO: PbO, or Pb(PbO:2), lead meta- 
plumbate. Bellucci and Parravano consider the hydrate to be 
Pb[Pb(OH).]. Similarly, red lead or minium Pb3;0,4 can be 
decomposed by certain acids into soluble plumbous salts and 
PbO, and is, therefore, regarded as 2PbO, PbOs, or Pb(PbO:): 
-lead orthoplumbate. No hydrate or hydrous form of Pb3Qxz is 
reported. 


1Se1weu: J. prakt. Chem., (2) 20, 200 (1879); BrLiuccr and PARRAVANO: 
Z. anorg. Chem., 50, 107 (1906). 


232 THE HYDROUS OXIDES 


While the hydrous oxides and the alleged hydrates of lead 
are not important commercially, the anhydrous compounds are 
widely used in the arts. ‘Thus, litharge is used in the manufac- 
ture of flint glass and as a glaze for earthenware. It is also used 
in making the very important pigments, white lead and min- 
ium, in the manufacture of plates for the lead accumulator, and 
as a “dryer” in oils. Minium, like litharge, is employed in 
making flint glass and battery plates, but its widest use is as a 
pigment. The peroxide has a strong oxidizing action and is 
frequently employed as an oxidizing agent. The mixture of 
nitrate and dioxide (oxidized red lead), obtained by heating red 
lead with nitric acid, is used in the manufacture of lucifer matches. 


CHAPTER IX 


THE HYDROUS OXIDES OF TITANIUM, ZIRCONIUM, 
AND THORIUM 


The hydrous oxides of titanium, zirconium, and thorium are 
always described as existing in both an alpha or ortha and a 
beta or meta modification. In every case the relationship 
between these two forms is the same as between the so-called 
a and 6 stannic oxides whose colloid chemistry has been consid- 
ered in detail in the preceding chapter. Accordingly, in this 
chapter will be given but a brief survey of this phase of the 
chemistry of the hydrous oxides under consideration. 


Hyprovus TITANIuM DIoxIDE 


The addition of ammonia or alkali hydroxide or carbonate to 
a cold solution of titanium dioxide in hydrochloric or sulfuric 
acid throws down the so-called orthotitanic acid as a voluminous 
white mass easily soluble in dilute acids. The product forms no 
hydrates! but is a typical hydrous oxide whose water content is 
determined by the method of precipitation and drying.? Dried 
in the air, the compound gives an x-ray interference pattern 
indicating its crystalline character.* If heated rapidly, the oxide 
exhibits the glow phenomenon; but as usual, if the heating is 
slow or the temperature is held for some time below the glow 
temperature, there is a gradual sintering and loss of surface - 
energy without incandescence.* 


1 CARNELLEY and WALKER: J. Chem. Soc., 53, 81 (1888). 

2 Rose: Pogg. Ann., 65, 507 (1844); Demoty: Compt. rend., 20, 325 (1845) 
Merz: Jahresber., 197 (1866); Turrscunw: Liebig’s Ann. Chem., 141, 111 
(1867). 

3 HeDvALL: Z. anorg. Chem., 120, 327 (1922). 


4See p. 79, 
233 


234 THE HYDROUS OXIDES 


Heating the acid solution of titanium dioxide to boiling pre- 
cipitates the typical betatitanic acid as a white powder. If 
obtained from hydrochloric acid solution, the oxide cannot be 
washed without undergoing peptization, forming a positive sol; 
but the oxide from sulfuric acid solution is not peptized by wash- 
ing, on account of the precipitating power of sulfate ion. The 
typical 8 oxide is distinctly less hydrous than the a; it does not 
exhibit the glow phenomenon on heating; and it is almost insol- 
uble in acids with the exception of concentrated sulfuric acid. 

Hydrous titania is only slightly soluble in alkalies, the solu- 
bility varying from 2 milligrams per 100 cubic centimeters in 
10 per cent sodium hydroxide to 120 milligrams in 100 cubic 
centimeters in 40 per cent potassium hydroxide. ‘The statement 
that NaeTiO; - 4H2O and K,T103- 4H.O! can be erystallized from 
alkaline solution of alkali titanate is obviously erroneous.? 

The a and 8 modifications of titanic acid are not chemical 
individuals but are hydrous oxides differing in the size and physi- 
cal character of the particles and in the amount of adsorbed 
water. The soluble highly gelatinous oxide ages gradually at 
ordinary temperatures*® and more rapidly at higher temperatures, 
forming a continuous series of products that approach the char- 
acter of the granular insoluble 8 oxide as a limit. This conclu- 
sion was confirmed by Morley and Wood by observations on the 
varying adsorption capacity for dyes® and on the varying solu- 
bility and peptizability by hydrochloric acid,® of the hydrous 
oxides prepared in different ways. ‘There seems no real justifica- 
tion for assuming, as Morley and Wood do, that the change in 
physical character of the gelatinous oxide on ageing is due to the 
formation of complex salt-like condensation products by the 
molecules of hydrous oxide functioning both as acid and base. 

Titanium Dioxide Sol and Jelly—Graham’ obtained a sol of 
hydrous titanium dioxide by dialysis of a 1 per cent solution of 


1 Demo.uy: Jahresber., 271 (1849). 

2 AUGER: Compt. rend., 177, 1802 (1923). 

3 WaGNER: Ber., 21, 960 (1888). 

4 LorrerMosER: Abegg’s ‘“‘Handbuch anorg. Chem.,” 3, (2), 883 (1909). 
5 J. Soc. Dyers Colourists, 39, 100 (1923). 

6 J. Chem. Soc., 125, 1626 (1924). 

7 Phil. Trans., 151, 213 (1861), 


TITANIUM, ZIRCONIUM, AND THORIUM 235 


the oxide in dilute hydrochloric acid. With more concentrated 
solutions, jellies are formed on the dialyzer during the purification 
process. ‘The water in an aged jelly can be replaced by alcohol, 
ether, benzene, glycerin, or concentrated sulfuric acid in the 
same way as in the corresponding silica jelly. 

More than a century ago, Rose! reported the formation of a 
soft titania jelly. He treated a fusion of titania and sodium car- 
bonate with hydrochloric acid, filtered the solution, and allowed 
it to stand, whereupon the hydrous oxide aged and precipitated 
out as a jelly. Later, Knop? obtained a jelly in an interesting 
way: A strong hydrochloric acid solution of magnetic oxide of 
iron was treated with tartaric acid and then neutralized with 
ammonia. ‘The iron remained in solution and the titania came 
down as a white precipitate. On filtering and attempting to 
wash the oxide, it swelled up in much the same manner as gelatin, 
forming a colorless transparent jelly which was transformed into a 
gelatinous precipitate by heating. Recently, Klosky and Mar- 
zano® prepared firm transparent jellies by neutralizing slowly 
an acid solution of titanium dioxide with the carbonates of 
sodium, potassium, or ammonium. 

Hydrous titanium dioxide probably finds its most important 
use as a mordant. If leather or textile goods are immersed in a 
solution of titanium salt and then steamed, the hydrous dioxide 
is precipitated. This adsorbs certain dyes forming permanent 
brilliantly colored lakes. As a mordant for alizarin orange, 
coerulein and alizarin blue, titania is superior to chrome.* For 
delicate fabrics, titanium salts of organic acids are employed in 
order to avoid the injurious action of mineral acids. Both tri- 
valent and tervalent salts are used for this purpose. 

If anhydrous titanium tetrachloride is sprayed into air, it 
takes up moisture, giving a dense smoke composed of fine particles 
of the solid hydrate, TiCl,- 5H2O. The chloride was used suc- 
cessfully during the war for producing smoke screens. In case 


1Gilbert’s Ann., 73, 76 (1823); cf. ProrpTEN: Liebig’s Ann. Chem., 237, 
213 (1887). 

2 Liebig’s Ann. Chem., 123, 351 (1862). 

3 J. Phys. Chem., 29, 1125 (1925). 

4 Barnes: J. Soc. Dyers Colourists, 12, 174 (1896); 35, 59 (1919); Ham- 
MEL: [bid., 20, 65 (1904). 


236 THE HYDROUS OXIDES 


the air is too moist, hydrolysis takes place, giving hydrochloric 
acid and hydrous titanium dioxide which forms a smoke, but with 
less obscuring power than the chloride hydrate. The cloud may 
be increased in moist air by the presence of ammonia which 
forms ammonium chloride. If the air is not quite moist, however, 
ammonia must be avoided, otherwise the chloride forms an 
ammonate, TiCl,- 6NH;, which has little obscuring power. On 
this account, it is usually planned to disperse the tetrachloride 
and ammonia separately. 

Precipitated titania makes a particularly good pigment in 
paints on account of its permanence, great opacity, and non- 
poisonous nature. It has been employed with barium sulfate 
in place of zinc oxide, giving a titanium lithopone. Titania 
paints are not affected by sea water; have no saponifying action 
on linseed oil; and have more than a third more covering power 
than white lead paints. 


OTHER OXIDES OF TITANIUM 


Titanium Monoxide.—The hydrous oxide of divalent titanium 
is thrown down as a black precipitate by adding hydroxyl ion 
to a solution of titanous salt. It is very unstable in the air, 
oxidizing first to blue hydrous titanium sesquioxide and finally 
to the white dioxide. 

Titanium Sesquioxide——The hydrous oxide of trivalent tita- 
nium has been variously described as black, dark blue, cherry 
red and brown red, depending upon the exact conditions of 
formation. It is prepared by digesting a solution of dioxide in 
hydrochloric acid with metallic copper at 20 to 40° until the 
solution attains a violet-blue color, followed by the addition of 
ammonia. It is also thrown down directly from titanium 
trichloride solution with ammonia. If the hydrous oxide is 
shaken with milk of lime in the presence of oxygen, it is oxidized 
to the dioxide and at the same time an equivalent amount of 
hydrogen peroxide is formed. In the same way, when the 
sesquioxide is oxidized by a solution of chromic acid in the 
presence of potassium iodide, or by potassium permanganate 
in the presence of tartaric acid, hydrous titanium dioxide is 
formed, and simultaneously, oxidation of the potassium iodide 


TITANIUM, ZIRCONIUM, AND THORIUM 237 


or tartaric acid is brought about.! These are typical cases of 
auto-oxidation.? 

Titanium Peroxide.—The addition of hydrogen peroxide to 
a neutral or acid titanium solution produces an intense yellow 
coloration, owing to the formation of a hexavalent titanium 
compound. Since the color is quite distinct, even in the presence 
of less than 0.01 per cent of titanium, the reaction affords a 
delicate test both for titanium and hydrogen peroxide.? If 
gelatinous titania is treated with an excess of hydrogen peroxide, 
it is converted into yellow titanium peroxide. The latter com- 
pound is best obtained by dropping titanium tetrachloride 
slowly into dilute alcohol; adding a large excess of hydrogen 
peroxide; and finally treating with ammonia, ammonium car- 
bonate, or alkali.t| The yellow hydrous oxide adsorbs salts 
very strongly, and so it is difficult to obtain pure.’ When freshly 
formed, the composition can be represented by the formula 
TiO;-2H20, but on drying over phosphorus pentoxide, it becomes 
a horny mass containing less oxygen than corresponds to a tri- 
oxide. The freshly precipitated hydrous peroxide appears to 
be considerably more soluble in alkali than the dioxide. This 
may be due in part to peptization, since the alkali peroxide 
solutions are instable, depositing an aged granular oxide in the 
course of a few days. It is usually assumed, however, that the 
alkali solutions contain alkali pertitanate.°® 


Hyprovus ZIRCONIUM DIOXIDE 


The most gelatinous form of hydrous zirconia is obtained by 
precipitating a solution of a zirconium salt with ammonia or 


1 Mancnort and Ricuter: Ber., 39, 320, 488 (1906); Mancuor and WIL- 
HELMS: Liebig’s Ann. Chem., 325, 105 (1902); Haser: Z. EHlektrochem., 7, 
441 (1900). 

2 ScHONBEIN: J. prakt. Chem., 98, 24 (1864); Trause: Ber., 26, 1471 
(1893); Van’r Horr: Z. physik. Chem., 16, 411 (1895); Eneier: Ber., 30, 
1669 (1897). 

3 Ricuarz and Lonnss: Z. physik. Chem., 20, 145 (1896); Haaser and 
GRINBERG: Z. anorg. Chem., 18, 37 (1898). 

4 Levy: Compt. rend., 110, 1368 (1890); Ann. chim. phys., (6) 25, 433 (1892). 

5 CLassEN: Ber., 21, 370 (1888); cf. WELLER: Ber., 15, 2592 (1882). 

6 MevrkorrF and PissarJewski: Ber., 31, 678, 953 (1898); Z. anorg. Chem., 
18, 59 (1898); cf. Bruty; Compt, rend., 172, 1411 (1921). 


238 THE HYDROUS OXIDES 


alkali hydroxide. The latter is adsorbed so strongly that the 
former must be employed if a pure gel is desired. This hydrous 
oxide, the so-called a zirconic acid, bears a marked resemblance 
to alumina both in its appearance and in its capacity to adsorb 
water and salts. Like alumina, also, it is almost entirely insolu- 
ble in water.!_ When dried at 100°, the gel is reported to be a 
monohydrate, ZrO.-H2.O or ZrO(OH).;? but van Bemmelen® 
showed that the minimum temperature necessary for attaining 
this composition depends on the previous history of the sample. 
Thus, the water content of van Bemmelen’s gels was not reduced 
to the point corresponding to a monohydrate until a temperature 
of 140° or more was reached. Between 140 and 200° the composi- 
tion was approximately constant. The latter observation might 
be taken to mean the existence of a definite hydrate of zirconia 
like the crystalline hydrates of beryllia and alumina. Van 
Bemmelen found, however, that the adsorption capacity for 
water and salts, of zirconia containing 1 mol of water was similar 
to that of hydrous alumina and beryllia and not like that of the 
crystalline hydrates. He concludes, therefore, that the water 
in the alleged hydrate of zirconia is adsorbed in capillaries and 
not chemically combined in the ordinary sense. This view 
receives strong support from recent investigations of the structure 
of zirconia sols and gels, using the method of x-ray interference. 
Haber and his pupils* find that hydrous zirconia possesses no 
crystalline structure whatsoever either when freshly precipitated 
or when thoroughly dried below 400°. As has been pointed out 
repeatedly in these pages, hydrous oxides, amorphous when first 
prepared, usually assume a microcrystalline form on ageing; 
and all the definitely established oxide-hydrates are crystalline. 
If zirconia forms an amorphous hydrate, it is an outstanding 
exception. 

When formed in the cold, the hydrous oxide is more gelatinous 
and more reactive than when formed in the hot. Either prep- 
aration heated to approximately 300° glows very brightly, pro- 


1 VENABLE and Betpen: J. Am. Chem. Soc., 20, 273 (1898). 

2 RuER: Z. anorg. Chem., 48, 297 (1905). 

3Z. anorg. Chem., 49, 125 (1906). 

4 Haper: Ber., 65 B, 1717 (1922); Boum and NicuassEn: Z. anorg. Chem., 
132, 1 (1924). 


TITANIUM, ZIRCONIUM, AND THORIUM 239 


vided the water content is not reduced below 1.9 per cent.! If 
the hydrous mass is heated rapidly above 300°, the glowing is 
accompanied by small explosions, caused, in all probability, by 
expulsion of some of the adsorbed water. It is an interesting 
fact that the oxide retains considerable water even after the glow- 
ing. Ruer looks upon the glow phenomenon as a manifestation 
of the transformation of ordinary zirconia into isomeric meta- 
zirconia; but Wohler? showed it to result from a sudden diminu- 
tion in surface energy accompanying the change from a gelatinous 
structure to a granular powder. 

Ruer prepared an aged hydrous zirconia by boiling down 
repeatedly a solution of zirconium oxychloride. The sol obtained 
by this process was precipitated by hydrochloric acid giving 
what Ruer called a metachloride. After centrifuging out the 
precipitate, it was peptized in water and thrown down again with 
ammonia. The hydrous oxide, still containing considerable 
chloride, was dried over caustic alkali and then heated to 100°, 
where its water content corresponded approximately to ZrOs.:- 
22H2O. On account of its relatively slight solubility in acids and 
its failure to glow on heating, Ruer believed it to be an isomeric 
form of zirconic acid which he designated metazirconic acid. 
Van Bemmelen found, however, that oxides prepared by Ruer’s 
method lost water continuously without any evidence of the 
existence of a hydrate. A composition corresponding to Ruer’s 
100° hydrate was obtained by van Bemmelen at 85°; and, at 
every observed temperature, the composition showed consider- 
able variation with different samples. Van Bemmelen showed 
further that Ruer’s metachloride was merely an aged zirconia 
with adsorbed chloride. By evaporating the oxychloride to 
dryness and replacing the water repeatedly, a product was 
obtained which retained but a trace of chloride. 

Prolonged boiling of hydrous zirconia in a medium possessing 
a slight solvent action gives a dense structure that is not only 
less reactive chemically but has a much lower adsorption capacity 
than the gelatinous precipitated oxide. This change in structure 
is a gradual process, involving the formation of a continuous 
series of products intermediate between the typical ortho and 


1 Van BEMMELEN: Z. anorg. Chem., 45, 83 (1905). 
* Kolloid-Z., 11, 241 (1918). 


240 THE HYDROUS OXIDES 


meta. oxides. It is unnecessary to start with zirconium oxy- 
chloride to prepare the so-called meta oxide. A sol of the 
ordinary oxide is aged by boiling, the amorphous particles gradu- 
ally becoming denser and at the same time tending to orient 
themselves into crystals.! 

Zirconium dioxide is the most important compound of zirco- 
nium from the technical viewpoint. Its very high melting point, 
low heat conductivity, low coefficient of expansion, low porosity, 
and high resistance to corrosion even at elevated temperatures, 
combine to make it an almost ideal refractory. The only 
difficulty is that very small amounts of certain materials modify 
its properties, and the removal of these is very expensive. ‘Thus, 
iron, which acts as a flux, can be removed entirely only by com- 
plete solution of the oxide in hydrofluoric acid. Moreover, the 
oxide prepared by igniting compounds such as the hydrous 
oxide or nitrate is a very loose powder that shrinks enormously 
when highly heated. Accordingly, high-temperature utensils 
such as muffles, crucibles, etc. must be made from zirconia which 
has been fused and subsequently ground to a powder. On 
account of its gelatinous character, hydrous zirconia makes a 
good binder for holding together the particles of fused zirconia, 
thus giving a paste that may be molded into the desired shape. 

In addition to its use as a refractory, anhydrous zirconia has 
been used for almost a century in connection with the problems of 
artificial ighting, because of the brilliant light emitted when it is 
heated to incandescence. The first -Welsbach mantles were 
made largely of zirconia, but this was later replaced by thoria, 
since the latter oxide glows at a much lower temperature. It is 
also employed for coating the lime or magnesia pencils in the 
Drummond light where it is distinctly advantageous, not only 
because of the brilliant light it emits but because it does not 
absorb carbon dioxide or moisture from the air as do lime and 
magnesia. ‘The Bleriot lamps used for automobile headlights | 
consist of zirconia rods heated to incandescence. Nernst 
employed rods of pure zirconia in his early attempts to obtain a 
means of illumination, by use of the electric current, which would 
be superior to the carbon filament lamp. Later, he obtained a 


1 Boum and NIcuLassEen: Z. anorg. Chem., 182, 6 (1924), 


TITANIUM, ZIRCONIUM, AND THORIUM 241 


more intense light with mixtures of the oxides of zirconium, 
thorium, yttrium, and sometimes cerium. 

Prepared in various ways, zirconia is used as a toilet powder, 
as a polishing powder, and as a substitute for bismuthyl] nitrate 
in the diagnosis of gastrointestinal disease by means of x-rays. 
For the latter purpose, it is distinctly advantageous because of 
its non-poisonous character. Zirconia is also used as an opacify- 
ing agent in enamels and a clouding agent in glass, instead of the 
more costly stannic oxide and the poisonous compounds of 
antimony and arsenic. Asa pigment, it possesses good covering 
power, mixes readily with paint vehicles, is permanent, and is 
unaffected by hydrogen sulfide, acids, or alkalies. 


ZIRCONIUM DIOXIDE SOLS 


Hydrolysis of Zirconium Salts.—Biltz! dialyzed a solution of 
zirconium nitrate for several days, obtaining a rather impure sol 
of hydrous zirconium dioxide which was slightly acid and gave 
a distinct test for nitrate. The sol possessed a positive charge 
which was neutralized by the addition of negative sols, the 
particles of opposite sign mutually precipitating each other. 

Ruer? dialyzed solutions of zirconium oxychloride both without 
heating and after heating for 2 hours. Like Biltz’s preparation, 
the sols were clear in transmitted light but cloudy by reflected 
light. Addition of sodium or ammonium chloride caused precip- 
itation, the amount required being less the more thorough the 
purification by dialysis. A transparent glass was obtained by 
evaporation on the water bath. The addition of 10 cubic centi- 
meters of N sulfuric acid to 2.5 cubic centimeters of sol containing 
0.015 gram of ZrO, gave a precipitate that dissolved in 144 hour 
provided the solution was not heated before dialysis; the pre- 
cipitate from the preheated solutions did not dissolve for approxi- 
mately 6 hours under similar conditions. This decrease in 
solubility was the manifestation of growth of primary particles 
which proceeded quite gradually at ordinary temperature but 
more rapidly at the boiling point. As we have seen, prolonged 
heating of the oxychloride gave a slightly hydrous mass, insoluble © 


1 Ber., 35, 4436 (1902); 37, 1100 (1904). 
2Z. anorg. Chem., 48, 282 (1905). 


242 THE HYDROUS OXIDES 


in both hydrochloric and nitric acid but readily peptized on 
washing with water. By dialysis of the so-called metachloride, 
Ruer obtained a milky white sol which left on evaporation an 
amorphous white residue instead of a glassy mass. The chlorine 
content was reduced to 0.026 atom Cl per mol of ZrOs. On 
account of the relatively slight adsorption capacity of the 
particles, the precipitation by electrolytes with univalent anions 
is readily reversible. 

Adolf and Pauli! attempted to establish the composition of 
equilibrium solutions of zirconium oxychloride of varying con- 
centrations up to 0.5 N by observations of the freezing-point 
lowering, conductivity, and directions of migration under elec- 
trical stress, as well as the hydrogen and chloride ion concen- 
trations using the hydrogen and calomel electrodes, respectively. 
The hydrolysis does not change materially with the dilution. 
The curves for hydrogen ion and chloride ion concentrations 
against the concentrations of ZrOCl, are S shaped and intersect 
each other at three points, so that, at very low and again at 
moderate concentrations, the hydrogen ion concentration appears 
to be greater than that of chloride, indicating the presence of 
complex ions containing zirconium. The osmotic concentration 
at the higher concentrations is less than the molar concentration 
of oxychloride and does not greatly exceed it even at the greatest 
dilutions. By subtracting from the total conductivity, the 
conductivity due to the hydrogen and chloride ions present, the 
conductivity due to the alleged complex zirconium ions is obtained. 
This appears to constitute a large part of the total conductivity 
and to vary with the concentration of oxychloride. These obser- 
vations are explained by assuming the formation of complex 
cations and anions such as 2[Zr(OH)4: ZrOCl,: ZrO] and 2[Zr- 
(OH),Cl.]’’.. Migration experiments indicate a migration of 
zirconium to both anode and cathode, more going to the anode 
than to the cathode when the hydrogen ion concentration exceeds 
the chloride ion concentration and vice versa. 

It is very difficult to make head or tail of the conglomeration of 
facts and speculations given in the preceding paragraph. ‘This 
difficulty increases when we reflect that Adolf and Pauli’s con- 
ductivity and electrometric measurements do not give what 


1 Kolloid-Z., 29, 173 (1921). 


TITANIUM, ZIRCONIUM, AND THORIUM 243 


they assumed them to give. Leaving out any complex ions, 
there are in any given solution: undecomposed oxychloride, 
hydrous zirconium dioxide, hydrogen ions, and chloride ions. 
The hydrous oxide adsorbs some undecomposed zirconium oxy- 
chloride and possibly stabilizes it to a certain degree. It also 
adsorbs both hydrogen ions and chloride ions in amounts depend- 
ing on the experimental conditions. The conductivity is due 
to the unadsorbed ions and to the hydrous oxide particles which 
_have adsorbed ions and which move with a velocity somewhat 
less than that of the free ions. Thus, the adsorbed ions contrib- 
ute to the conductance of the solution, but they behave abnor- 
mally as regards electrometric measurements. Adsorbed chloride 
ion gives no test with silver nitrate, and its effect on the calomel 
electrode will be negligible. To assume that all the hydrogen 
and chloride ions which do not show up in electrometric measure- 
ments exist in complex ions will necessarily lead to erroneous 
conclusions. Until we know definitely what Adolf and Pauli’s 
conductivity and electrometric measurements actually mean, 
it seems idle to speculate as to the real nature of solutions of 
zirconium oxychloride, whether dialyzed or undialyzed. Adolf 
and Pauli assume the existence of complex ions in sols where the 
ratio ZrO, to Clis 3 or 4:1. This would seem to be a far-fetched 
assumption in a sol such as Ruer’s, where the ratio is 40: 1 or more. 
A very satisfactory sol was prepared by Rosenheim and 
Hertzmann! by the dialysis for a week of a 1.5 per cent solution of 
zirconium acetate. The colloid was perfectly clear in both trans- 
mitted and reflected light and contained but a trace of acetate. 
Heating on the water bath converted the sol into a clear trans- 
parent jelly. It was very sensitive to the action of electrolytes, 
dilute potassium chloride precipitating it quantitatively. 
Peptization of Hydrous Zirconia.—Miiller? prepared sols both 
by adding freshly precipitated and washed zirconia to a solution 
of zirconium nitrate and by adding ammonia drop by drop to 
the nitrate solution until the precipitate first formed just failed 
to redissolve. Evaporating the sol to dryness gave a gummy 
residue that swelled in water and was then repeptized. The 
oftener this process was repeated, the smaller the nitrate content 


1 Ber., 40, 813 (1907). 
2Z. anorg. Chem., 52, 316 (1907). 


244 THE HYDROUS OXIDES 


became. The sols were precipitated by low concentrations of 
electrolytes containing multivalent anions. Zirconium sulfate 
like the nitrate, peptizes hydrous zirconia. Hauser' showed 
conclusively that the products of the peptizations are sols and 
not basic salts, ZrOSO,and ZrO(NOs)e, as asummed by Berzelius? 
and Paykull.’ 

Szilard* added ammonia to a zirconium nitrate solution and 
washed the resulting gel thoroughly, using the centrifuge, until 
complete peptization took place. In this way, a highly purified 
zirconia sol was obtained which was quite sensitive to the action 
of electrolytes, carbon dioxide from the air being sufficient to 
induce coagulation. Szilard® also peptized the purified gelati- 
nous oxide with the nitrates of zirconium, thorium, and uranyl, 
obtaining sols similar to those of Miiller. — 

A zirconia sol of suitable concentration is converted into a 
jelly by adding enough electrolyte to cause slow coagulation. 
The jelly can be broken up by shaking, giving a sol which will 
set again to a jelly; but the process cannot be repeated very often 
without throwing down a gelatinous precipitate.°® 


ADSORPTION BY HYDROUS ZIRCONIA 


On account of its highly gelatinous character, hydrous zirconia 
possesses a marked adsorption capacity for many substances.7 
The taking up of iodine and ammonia by the hydrous oxide care- 
fully purified by dialysis does not follow the ordinary adsorption 
rule. Instead, the amount taken up increases with the concen- 
tration of the solutions without approaching a constant value, 
thus indicating the formation of a solid solution. Colloidal 
solutions of ferric oxide, molybdenum blue, zirconium, and silver 
are quickly decolorized by shaking with a paste of hydrous 
zirconia. The blue starch-iodine sol is taken up, giving a blue 
zirconia gel which is decolorized by heating and becomes blue 


1Z. anorg. Chem., 54, 208 (1907). 

2 Pogg. Ann., 4, 117 (1825). 

3 Ber., 6, 1467 (1873). 

4 J. chim. phys., 5, 488 (1907). 

® J. chim. phys., 5, 636 (1907). 

6 SCHALEK and Szravari: Kolloid-Z., 38, 326 (1923). 
7 WEDEKIND and RHEINBOLDT: Ber., 47, 2142 (1914), 


TITANIUM, ZIRCONIUM, AND THORIUM 245 


again on cooling, just as the original sol. Colloidal Congo red is 
strongly adsorbed, giving a blue adsorption compound which is 
converted into a red salt by warming.! The last-mentioned 
phenomenon has been observed in a number of instances by 
Wedekind and Wilke.? Thus, arsenic acid is adsorbed in the 
cold by hydrous zirconia; but on standing or boiling, Zr(HAsOu.)> 
is formed. A similar thing was observed with phosphoric acid; 
but adsorption only takes place with the following acids: arseni- 
ous, monochloracetic, hydrochloric, and perchloric. Obviously, 
the tendency to form salts following adsorption is not a question 
of the strength of the acids. Zirconia gel rapidly catalyzes the 
decomposition of hydrogen peroxide, especially in concentrated 
solutions; but the removal of hydrogen peroxide by the gel 
during very short periods of contact with dilute solutions can be 
represented by the usual adsorption equation. After prolonged 
contact, the hydrogen peroxide in solution is almost completely 
decomposed, but large amounts remain in the gel. Not all the 
peroxide taken up by the gel can be titrated by permanganate in 

8 per cent sulfuric acid. ‘This is taken to indicate the formation 
of a complex peroxide following the initial adsorption. 

The adsorption capacity of hydrous zirconia for certain dyes 
suggests the use of zirconium salts as mordants? and in the prepara- 
tion of lac dyes.* For these purposes the hydrous oxide poss- 
esses no properties that are distinctive and so it finds but limited 
application. 


Hyprovus ZIRCONIUM PEROXIDE 


A hydrous peroxide of zirconium was first obtained by adding 
ammonia to a solution containing zirconium sulfate and hydrogen 
peroxide.’ Such gels contain both dioxide and peroxide; but 
Bailey® added hydrogen peroxide alone to solutions of zirconium 
salts, obtaining gelatinous precipitates which analyzed approxi- 


1Cf. Bayuiss: Chem. Zenir. II, 1095 (1911). 

2 Kolloid-Z., 34, 83, 283; 35, 23 (1924). 

3 BarRNEs: J. Soc. Chem. Ind., 15, 420 (1896); Wenarar: Fdrber Ztg., 25, 
277 (1914). 

4 ScHEURER and BrytiuskI: Bull. soc. ind. Mulhouse, 68, 124 (1898). 

5 Curve: Bull. soc. chim., (2) 48, 57 (1885). 

6 J. Chem. Soc., 49, 149, 481 (1886); Proc. Roy. Soc., 46, 74 (1890); cf. 
Hermann: J. prakt, Chem., 97, 331 (1866). 


246 THE HYDROUS OXIDES 


mately for ZrO3;:3H.O0 when dried over phosphorus pentoxide 
at ordinary temperature. The oxide loses oxygen on heating, 
the composition approaching Zr.O; at 100°. Pissarjewsky! 
obtained hydrous ZrO; by electrolyzing a sodium chloride solu- 
tion in which hydrous ZrO. was suspended. Irrespective of 
the method of preparation, the higher oxide behaves as a true 
peroxide, giving off oxygen on standing and yielding hydrogen 
peroxide when treated with dilute sulfuric acid. The gelatinous 
oxide is fairly soluble in alkali, and perzirconates are said to form. 


Hyprovus THORIUM DIOoxIDE 


The ordinary gelatinous form of hydrous thorium dioxide is 
precipitated by adding ammonia or alkalies to a cold solution of 
thorium salt. The gel is readily soluble in mineral acids but is 
insoluble in alkalies. The anhydrous oxide obtained by igniting 
thorium nitrate, sulfate, or the hydrous oxide is not attacked by 
acids, whereas that prepared by ignition of the oxalate under 
suitable conditions is a loose insoluble powder which is rendered 
soluble by boiling to dryness with hydrochloric or nitric acid. 
As in the case of zirconia, people have assumed that the product 
obtained by ignition of thorium oxalate is a meta oxide and that 
the product of the action of hydrochloric acid, say, is a meta 
chloride.? These assumptions are erroneous, since the chlorine 
content of the alleged compound varies through wide limits and 
the solution in water is a typical case of sol formation. 

Thoria Sols.—By dialyzing a 14 per cent solution of thorium 
nitrate for several days, Biltz? obtained a dilute, water-clear 
thoria sol containing a small amount of nitrate ion. This sol 
is stabilized by preferential adsorption of Th’*** and H’ ions and 
so is precipitated by suitable amounts of negatively charged sols. 
Under the influence of electrical stress, the colloidal particles 
migrate to the cathode, where they precipitate as a jelly contain- 
ing bubbles of gas. Miiller* obtained similar sols containing as 
much as 15 grams ThO; in 100 cubic centimeters by peptizing 


1Z. anorg. Chem., 25, 378 (1900); 31, 359 (1902). 

2 CLEVE: Bull. soc. chim., (2) 21, 115 (1874); Stevens: Z. anorg. Chem., 
27, 41 (1901). 

3 Ber., 35, 4436 (1902); 37, 1095 (1904). 

4 Ber., 39, 2857 (1906); Z. anorg. Chem., 57, 314 (1908), 


TITANIUM, ZIRCONIUM, AND THORIUM 247 


freshly precipitated and washed hydrous thoria with thorium 
nitrate, hydrochloric acid, aluminum chloride, ferric chloride, 
and uranyl nitrate. The sols are slightly cloudy, but they can be 
boiled without precipitating. The particles in the newly formed 
sols are completely amorphous; but the ageing which accom- 
panies boiling, results gradually in the appearance of a crystal- 
line structure detectable by x-ray analysis.!_ Evaporating to 
dryness gives a glistening brittle varnish-like residue which 
swells in water and finally is dispersed into a distinctly opalescent 
sol. All of Miuller’s sols are quite sensitive to the action of 
electrolytes, particularly those with multivalent precipitating 
ions. By shaking with benzene,’ the hydrous oxide is precipitated 
at the benzene-water interface as a gel containing bubbles of air. 
The amount of electrolyte required to peptize a given quantity 
of hydrous oxide depends on the history of the sample. Szilard® 
peptized the fresh oxide precipitated from thorium nitrate solu- 
tion with ammonia, by thorough washing to remove the excess 
of ammonium nitrate. If the oxide is allowed to age even under 
water, it is not peptized by washing. As already noted, ignition 
of the hydrous oxide renders it non-peptizable; and the oxide 
from thorium oxalate is peptized only after boiling to dryness 
with a mineral acid, such as hydrochloric. By the latter process, 
Bahr‘ first prepared a so-called metachloride which was described 
as forming an opalescent solution in water. By repeated evapo- 
ration and repeptization in water, Cleve® obtained a preparation 
containing less than 1 per cent of chlorine. Cleve also observed 
the instability of the supposed solutions in the presence of various 
electrolytes. Stevens® found that the hydrous oxide, ignited 
until it is completed’ dehydrated, no longer forms a ‘‘soluble”’ 
chloride with hydrochloric acid. He attributed the observed 
variation in the thorium-chlorine ratio in the alleged compounds 
to the existence of several oxychlorides.’ Moreover, the failure 


_ 1B6um and Nicuassen: Z. anorg. Chem., 182, 6 (1924). 
2 WINKELBLECH: Z. angew. Chem., 19, 1953 (1906). 
3 J. chim. phys., 5, 488, 636 (1907). 
4 Liebig’s Ann. Chem., 132, 227 (1864). 
5 Bull. soc. chim., (2) 21, 117 (1874). 
6 Z. anorg. Chem., 27, 41 (1901). 
7 Cf. Wyrousorr and VERNEUIL: Compt. rend., 127, 863 (1898). 


248 THE HYDROUS OXIDES 


to obtain a test for chloride with silver nitrate was believed to 
furnish conclusive proof of compound formation. 

All of the observations on thoria gels and sols are readily inter- 
preted in the light of van Bemmelen’s' investigations on hydrous 
zirconia. <A freshly formed thoria gel is easily soluble in acids 
and is readily peptized by small amounts of certain acids and 
salts. On standing, the structure becomes more dense, and the 
solubility and ease of peptization fall off. Ignition of thorium 
oxalate under favorable conditions gives a very loose fine powder 
which can be peptized by acids. To get a product that is com- 
pletely peptizable, the temperature of ignition of the oxalate 
must not exceed 700°, and the product thus formed must not be 
kept at temperatures above 500° for any considerable time. 
Ignition at very high temperatures causes sintering of the par- 
ticles so that little or no peptization results, even with the strong- 
est acids. The transformation from the easily peptized gel to 
the non-peptizable granules is a continuous process which does 
not involve the formation of definite isomers. 

Kohlschiitter and Frey? showed that peptization of the 
solid oxide by acids is accompanied by a decrease in the volume 
of the colloidal system, which is probably to be explained by the 
porous nature of the oxide. During peptization the electrical 
conductivity and the titer of the acid decrease, and the presence 
of thorium salts in the solution can be proved analytically; but 
peptization and solution do not proceed parallel to one another. — 
The existence of the strongly adsorbed Th’*’’ and H’ ions causes 
the particles to be positively charged. The sol formed with 
hydrochloric acid is a hydrous oxide containing more or less 
adsorbed chloride depending on the conditions of formation. 
Adsorbed chloride gives no test with silver nitrate, and free 
chloride may not be precipitated owing to adsorption of silver 
chloride by the hydrous oxide, which prevents agglomeration 
into particles large enough to settle.* 

Thorium dioxide finds its most important use in the manu- 
facture of the incandescent gas mantle. For this purpose, the 


1Z. anorg. Chem., 49, 125 (1906). 

2Z. Elektrochem., 22, 145 (1916). 

3 Cf. Hantzscu and Dsscu: Liebig’s Ann. Chem., 328, 38 (1902); Rumr: 
Z. anorg. Chem., 48, 85 (1905). | 


TITANIUM, ZIRCONIUM, AND THORIUM 249 


mantle fabric is impregnated with a mixture of the nitrates of 
thorium and cerium which will yield the oxides in the proportion 
of 99 per cent thoria and 1 per cent ceria. On ignition, thorium 
nitrate expands at least tenfold, but cerium oxide has approxi- 
mately the same volume as the nitrate crystals from which it is 
prepared. Accordingly, the mixture of oxides in the used mantle 
is highly porous, the ratio of ceria to thoria by volume being 
about 1 to 999. The emissive power! of pure thoria is relatively 
low even though it reaches a high temperature in the flame. 
Ceria, on the other hand, has a high emissive power in the visible 
spectrum, but the energy of radiation is so great that the tem- 
perature of a pure ceria mantle does not rise sufficiently high to 
give the desired luminosity. On adding ceria to pure thoria 
the emissive power is increased but the temperature of the mantle 
is lowered.2. The maximum luminous efficiency is reached when 
the rise of visible emission due to ceria is just balanced by the 
drop in the temperature of the mantle caused by the increased 
radiation. 

It is an interesting coincidence that the mixture of thoria and 
ceria most efficient in catalyzing the combination of electrolytic 
gas appears to be the same as that which gives the maximum 
luminous efficiency in the Welsbach mantle. This means that 
a higher temperature will be reached and consequently a larger 
radiation of visible energy by the combustion at the surface of 
the Welsbach mixture than at the surface of any other thoria- 
ceria catalyst. There is no doubt but that surface combustion 
takes place with the Welsbach mantle and the Drummond light. 
This is evidenced by the slow decay of the light in the Welsbach 
mantle and the much more rapid decay of the light in the Drum- 
mond light, owing to sintering. It is not known definitely 
whether the ceria and thoria form a solid solution as has been 
suggested ;> nor is it known just what role the ceria plays in pro- 
moting the combustion of electrolytic gas. Swan* suggests that 


1 Rusens: Ann. Physik, (3) 20, 583 (1906); Ives, Kinaspury, and Karrer: 
J. Franklin Inst., 186, 401, 585 (1918). 

2 Popszus: Z. Physik., 18, 212 (1923). 

3 Swan: J. Chem. Soc., 125, 780 (1924). 

4 STEINMETZ: “Radiation, Light, and [lumination,” 92 (1909). 

5 WuitTE and Traver: J. Soc. Chem. Ind., 21, 1012 (1902), 


250 THE HYDROUS OXIDES 


it may act as an oxygen carrier! or may increase the electron 
emission of thorium and thus bring about a greater ionization 
of the gases. 

Small pencils of the Welsbach mixture of thoria and ceria 
become brilliantly luminous like the incandescent mantle when 
heated to a moderate temperature. Lamps of this kind are of 
use for searchlight and projection lanterns for moving pictures 
wherever the electric current is not available. 

In addition to its use in artificial lighting and as a refractory, 
thoria has been employed for defining the digestive tract in 
clinical examinations by means of x-rays.” It is also used as a 
catalyst in the synthesis of many organic compounds. For 
example both symmetrical and unsymmetrical ketones are 
prepared directly from monocarboxylic acids;* alcohols are con- 
verted into ethers and olefines, depending on the temperature 
employed; and ammonia and alcohols yield olefines and primary 
amines at 360°.4 


Hyprovus THoRIUuM PEROXIDE 


Hydrous thorium peroxide is thrown down in a gelatinous form 
by adding hydrogen peroxide to a solution of thorium acetate 
sulfate or nitrate.’ The gel adsorbs acids quite strongly; hence, 
it is very difficult to obtain in a pure state. Wyrouboff and 
Verneuil attempted to avoid this contamination by carrying 
out the precipitation in the presence of an excess of ammonia; 
but under these conditions, the precipitate contained nitric acid 
resulting from the action of hydrogen peroxide on the ammonia. 
The peroxide is formed by the action of hydrogen peroxide on 
hydrous thorium dioxide and also by electrolysis of a sodium 
chloride solution in which the dioxide is suspended. The 
latter method of formation indicates that the product is a true 


1 Mryer and Anscniirz: Ber., 40, 2639 (1907). 

2 KaESTLE: Miinch. med. Wochschr., 56, 919 (1909). 

3 SENDERENS: Compt. rend., 148, 927 (1909); Ka@uuErR: Bull. soc. chim., 
(4) 15, 647 (1914). 

4 MaruHE: Chem. Zig., 34, 1173 (1911). 

5 WyRouBOFF and VERNEUIL: Ann. chim. phys., (8) 6, 441 (1906). 

6 Lecog DE BoisBAuDRAN: Compt. rend., 100, 605 (1885); Cumve: Bull. 
soc. chim., (2) 48, 53 (1885). 


TITANIUM, ZIRCONIUM, AND THORIUM 251 


peroxide and not an addition compound of hydrous thorium- 
dioxide and hydrogen peroxide. 

The freshly prepared peroxide appears to be hydrous TheO;, 
but this is quite unstable, going over on standing to the much 
stabler ThO3.! Dilute sulfuric acid reacts with it, giving hydro- 
gen peroxide; and strong sulfuric acid gives ozone. Unlike the 
corresponding compounds of titanium and zirconium, it is not 
attacked by alkalies. 


1 PISSARJEWSKY: Z. anorg. Chem., 31, 359 (1902); 25, 378 (1900). 


CHAPTER X 
THE HYDROUS OXIDES OF THE RARE EARTHS 


The term rare earths is applied to a group of closely related 
trivalent metals forming basic oxides with oxalates insoluble in 
dilute mineral acids. The rare-earth group includes scandium, 
yttrium, and lanthanum, together with all the elements between 
cerium, atomic number 58, and lutecium, atomic number 71, 
inclusive. These elements are frequently divided into three 
families, the basis for the arbitrary classification being the solu- 
bility of the double alkali sulfates.1 The elements of the cerium 
family, scandium, lanthanum, cerium, praseodymium, neody- 
mium, and samarium, form quite insoluble double sulfates; and 
the elements of the yttrium family, dysprosium, holmium, 
erbium, thulium, yttrium, ytterbium, and lutecium, form quite 
soluble double sulfates. On the border line between these two 
families are the terbium family elements, europium, gadolinium, 
and terbium, whose double sulfates are but moderately soluble. 
The hydrous oxides of the cerium group are the best known 
and will be considered separately, beginning with hydrous ceric 
oxide. 


Tur Hyprovus OXIDES OF THE CERIUM FAMILY 


Hydrous Ceric Oxide.—Cerium differs from all the other 
members of the rare-earth family in forming a definite series of 
ceric salts derived from the most stable oxide of cerium, CeQOs. 
It is only as a trivalent metal that cerium exhibits the proper- 
ties of a typical rare earth. 

Hydrous ceric oxide is precipitated as a yellowish highly 
gelatinous mass by adding ammonia or alkali to a solution of 
ceric salt. It is also formed by oxidizing hydrous cerous oxide 
suspended in water, either by the oxygen of the air or by adding 
an oxidizing agent such as chlorine, bromine, alkali hypochlorite, 

1 Ursain: Ann. chim. phys., (7) 119, 184 (1900), 

252 


THE HY DROUS OXIDES OF THE RARE EARTHS 253 


or sodium peroxide. Like most gelatinous precipitates, it 
adsorbs alkali salts and hydroxide strongly and so is best obtained 
pure by precipitating cold ceric ammonium nitrate with ammonia, 
allowing the washed precipitate to dry partially, and finally 
rewashing to remove all ammonium nitrate. The precipitate 
dried over potassium hydroxide has the formula CeO2: 1.5H20,! 
but it is-altogether unlikely that this is a definite hydrate. 
Indeed, B6hm and Niclassen? found the ammonia precipitated 
oxide to be crystalline, the x-radiogram showing it to be CeQOs. 
On the other hand, the hydrous gel obtained by dialysis of 
ceric ammonium nitrate is amorphous. The hydrous oxide 
gives up a great deal of adsorbed water on standing and is 
transformed into a fibrous or granular mass. If dried below 120°, 
it dissolves in acids and alkalies, but the ignited oxide is quite 
insoluble.* 

Although the highly dispersed gelatinous oxide free from 
praseodymium,‘ is white;> when calcined at a high temperature, 
it assumes a citron-yellow color, becoming white or a lighter 
yellow again on cooling. The tint assumed on ignition depends 
on the mode of preparation; that obtained by igniting the 
hydrous oxide is darker than that from the sulfate; but according 
to Wyrouboff and Verneuil,® the tint of neither is definite enough 
to be described other than as a shade of white. Spencer’ 
attributes the yellow color to polymerization, and Sterba® sug- 
gests that it may be due to a higher oxide. There seems to be 
no experimental justification for either of these assumptions, 
and I am inclined to believe that the color assumed on heating 
is due to coalescence of particles which appear white in a finer 
state of subdivision. It is well known that zinc oxide is yellow 
when hot, due to coalescence of particles; but disintegration 


1 Wryrovusorr and VERNEUIL: Ann. chim. phys., (8) 9, 289 (1906); Ram- 
MELSBERG: Pogg. Ann., 108, 40 (1859); Erx: Z. Chem., (2) 7, 100 (1871); 
cf., however, CARNELLEY and Waker: J. Chem. Soc., 58, 59 (1888). 

2Z. anorg. Chem., 132, 1 (1924). 

3 MENGEL: Z. anorg. Chem., 19, 71 (1899). 

4 Wir: Chem. Ind., 19, 156 (1896). 

5 Cf., however, BRAUNER: Z. anorg. Chem., 34, 207 (1903). 

6 Ann. chim. phys., (8) 9, 356 (1906). 

7 J. Chem. Soc., 107, 1272 (1915); Muymr: Z. anorg. Chem., 37, 378 (1903). 

8 Compt. rend., 188, 221 (1901); Ann, chim. phys., (8) 2, 193 (1904). 


254 THE HYDROUS OXIDES 


takes place on cooling, accompanied by a return to the white 
color. However, a thoroughly sintered mass of zinc oxide 
remains yellow indefinitely, even on cooling.! Similarly, the 
citron-yellow color of hot hydrous ceric oxide becomes white or 
light yellow on cooling, depending on the time and temperature 
of ignition. The oxide has been suggested as a yellow opacifying 
agent for glass and enamel.? 

Cerium salts may be used more or less successfully for tanning 
leather? and as a mordant in dyeing cotton.4 In both of these 
processes, the hydrous oxide plays an important role. By far 
the most important use to which the oxide has been put is in 
the manufacture of incandescent mantels. This application 
has been referred to already, in connection with thorium oxide. 

The precipitate of hydrous ceric oxide obtained in the cold by 
adding sodium peroxide to a solution of cerous salt is reddish 
brown in color; but on boiling, oxygen is evolved and the color 
disappears.® The red-brown color may be due to a higher oxide 
of cerium, possibly hydrous CeQO;° which is instable at 100°. 

Ceric Oxide Sols —Hydrous ceric oxide sol is best prepared by 
dialysis of a solution of ceric ammonium nitrate.’ The sol may 
be evaporated to dryness on the water bath, giving a 
gummy mass which goes into colloidal solution again on shaking 
with water. There is no evidence of crystal structure in the 
hydrous oxide formed in’this way.® 

Like hydrous chromic oxide and ferric arsenate, the sol pre- 
pared by dialysis of ceric ammonium nitrate sets to a firm jelly 
if the dialysis is carried too far. This is particularly noticeable 


1Farnau: J. Phys. Chem., 17, 653 (1918). 

2 RICKMANN and Rappe: British Patent 203773 (1908). 

3 KitnER: Gerber, 37, 199, 213 (1911); GaRELLI: Collegiwm, 418 (1912); 
PareEnzo: [bid., 121 (1910). 

4 Matscuak: Chem. Ind., 21, 150 (1898); Wirt: Jbid., 19, 156 (1896); 
WarGNneErR and Mtuumr: Z. Farben- u. Textil Chem., 15, 290 (1903); BAskER- 
VILLE and Fouts: J. Soc. Chem. Ind., 28, 104 (1904). 

6 MmNGEL: Z. anorg. Chem., 19, 71 (1899). 

6 Lecog DE BorsBAUDRAN: Compt. rend., 100, 605 (1885); CiEvE: Bull. 
soc. chim., (2) 48, 53 (1885); Knorr: Z. angew. Chem., 11, 687, 717 (1897). 

7 Biuttz: Ber., 36, 4431 (1902). 

8 B6uM and NicuassEN: Z. anorg. Chem., 182, 6 (1924), 

9 Farnav and Pautt: Kolloid-Z., 20, 20 (1917), 


THE HYDROUS OXIDES OF THE RARE EARTHS 259 


if the initital concentration of the peptizing agent, nitric acid, 
falls below a critical value that is determined in part by the pres- 
ence in the sol of the precipitating electrolyte, ammonium 
nitrate.! Thus a jelly returns to the sol conditions if shaken up 
with a quantity of fresh undialyzed sol; and the concentration of 
electrolyte necessary to precipitate the hydrous oxide as a jelly 
is increased by adding a small amount of nitric acid. 

Kruyt and van der Made studied the effect of different elec- 
trolytes on the nature of the precipitate, obtaining stable jellies 
that do not contract in some instances and undergo rapid synere- 
sis or coagulation in others. As shown,’ this is purely a question 
of rate of precipitation of the sol, which in turn, is determined 
by the concentration of added electrolyte. Jellies with almost 
identical properties should result with any precipitating elec- 
trolyte that does not react with the particles, provided the con- 
centration is such as to allow a suitable slow rate of precipitation. 
The order of concentration of ions necessary for jelly formation 
fieeeenours is. br > ClO, > Cl > NO; > CNS > I > SOQ, > 
HPO,. Alcohol likewise decreases the stability of the sol and in 
concentrations of 40 to 50 per cent precipitates it as a jelly. 

Like the precipitated hydrous oxide, the primary particles in 
the sol condition coalesce and lose water more rapidly than is 
usual with sols of the hydrous oxides. This ageing is readily 
followed viscosimetrically, since the loss of adsorbed water by 
the dispersed particles is accompanied by a marked decrease in 
viscosity. A solution of ceric ammonium nitrate dialyzed short 
of the appearance of any gel on the dialyzer gives a viscosity- 
time curve having the general form represented in Fig. 15. 
This curve is for a sol containing 1.28 grams CeO, which was 
prepared by continuous dialysis for 30 hours of a 6 per cent 
cerlum ammonium nitrate solution. The initial increase in 
viscosity is a manifestation of gelation; with a relatively strong 
sol, this may proceed to the point where the time of flow can no 
longer be measured, followed in the course of a few weeks by a 
decrease in viscosity until the value of an aged sol is reached. If 
gelation has already started in the dialyzer before the viscosity 
measurements are begun, the maximum in the viscosity curve is 

1Cf. Kruyt and vAN DER Maps; Rec, trav, chim., (4) 42, 277 (1923). 

2 See p. 26, 


256 THE HYDROUS OXIDES 


missed. Nor is it observed after the sol has been heated to 50° 
which causes rapid ageing, or when the concentration of the sol is 
too low to admit of marked coalescence of the hydrous particles. 
Thus, the character of a hydrous ceric oxide sol is influenced 
to a marked degree by relatively slight variations in the method 
of dialysis, concentration, temperature, and time. The particles 
of a heated sol or one aged by long standing in the cold are no 
longer sufficiently hydrous to give a jelly on precipitation, at 
least in concentrations as low as 1.5 per cent CeQs. 


2.00 





on 
e) 





mM 
on 





Relative Viscosity 


Time,days 


Fig. 15.—Change in viscosity of CeOzg sols with time. 


Farnau and Pauli! added to a fresh sol insufficient salt to pro- 
duce coagulation and observed an immediate drop in the viscosity 
of the sol, followed by a gradual increase in viscosity, the final 
result being a jelly; with still less salt, the initial diminution in 
viscosity was followed by an increase to a maximum and there- 
after by a slow decrease as indicated by the dotted curve in Fig. 
15. 8 and y rays from radium act on the sol in much the same 
manner as electrolytes. Prolonged action produces a firm stable 
jelly, while shorter action results in a viscosity-time curve readily 
distinguished from the electrolyte curve by a much steeper rise 
and fall on opposite sides of the maximum, as shown by the 
results of observations of Farnau and Pauli represented in Fig. 16. 


1 Kolloid-Z., 20, 20 (1917). 


THE HYDROUS OXIDES OF THE RARE EARTHS = 257 


The sols used in these experiments contained 0.96 per cent CeO. 
and had already begun to decrease in viscosity when the measure- 
ments were started. On the fifth day the sol was subjected, for 
13 hours only, to the action of 8 rays from radium. This brought 
about a sharp rise in viscosity, which reached a maximum on the 
twentieth day, followed by a sharp fall. On the fifty-first day the 


1.9 
4 =Rays Applied 
t=Rays Removed 











Vis cosity 








\ hs 
: ig 


0 10 20 50 40 50 60 
Time, days 
Fic. 16.—Effect of B-rays from radium on CeO: sol. 


8B rays were applied continuously until gelation took place on 
the fifty-fifth day. 

Under the influence of a suitable amount of electrolytes or 
prolonged action of radiations, the charge on the particles is 
neutralized. This is apparently accompanied by a loss of 
adsorbed water and a consequent lowering of the viscosity, 
gradual under the influence of @ and y rays but immediately when 
an electrolyte is added. The subsequent increase in viscosity 


258 THE HYDROUS OXIDES 


is due to aggregation of the electrically neutral particles forming 
a jelly. The attainment of a maximum viscosity and the sub- 
sequent fall, when the added electrolyte is small in amount or’ 
the time of exposure to the rays is comparatively brief, is attrib- 
uted by Farnau and Pauli to the peptizing action of electrically 
charged particles entangled in the jelly. Since hydrogen ion is 
the stabilizing ion of the sol, observations of the changes in the 
hydrogen ion concentration might throw some light on the 
anomalous behavior during the ageing process. 

Kruyt and van der Made! peptized hydrous ceric oxide with 
dilute hydrochloric acid; but an excess of peptizing agent was 
‘required. Wyrouboff and Verneuil? decomposed cerium oxalate 
at as low a temperature as possible and heated the oxide with 2 
per cent nitric acid on the water bath. The resulting product, 
dried at 100°, was assigned the formula (CeOe)4- 4HNO3. It 
dissolved in water and on dialysis gave a precipitate that was 
represented as (CeOz)40° 10H2O. The soluble product was not 
a definite compound as Wyrouboff and Verneuil supposed, 
and the apparent solution was simply an aged CeQOz sol peptized 
by nitric acid. 

Hydrous Cerous Oxide.—This compound is obtained as a pure 
white,*® gelatinous precipitate by treating a cerous salt solution 
with ammonia or alkali in the absence of air. It oxidizes readily 
in the air especially in the presence of alkali,* the color changing 
to violet and finally light yellow, owing to the formation of 
hydrous CeO,. A similar color is obtained by heating ceric 
carbonate, nitrate, oxalate, or oxide in hydrogen. As one should 
not expect a mixture of two light bodies to be violet, the colored 
body is probably a cero-ceric oxide to which Chase® and Meyer® 
assign the formula C407 or 2CeQ2 - Ce2O3; and Wyrouboff and 
Verneuil’ the formula Ce7;Oy. or 3CeQO2- 2Ce203. The latter 


1 Rec. trav. chim., (4) 42, 278 (1923). 

2 Compt. rend., 124, 1300 (1897); Bull. soc. chim., (3) 17, 679 (1897). 

3 Dennis and Maazn: J. Am. Chem. Soc., 16, 649 (1894); DaAmiEns: 
Ann. chim., (9) 10, 137 (1918). 

4 SpENcER: J. Chem. Soc., 107, 1265 (1915). 

5 J. Am. Chem. Soc., 39, 1576 (1917). 

6 Z. anorg. Chem., 37, 378 (1903); cf. SteRBA: Ann. chim. phys., (8) 2, 
193 (1904). 

7 Ann. chim. phys., (8) 9, 289 (1906); Compt, rend,, 128, 501 (1899). 


THE HYDROUS OXIDES OF THE RARE EARTHS — 259 


investigators obtained the violet product directly by adding 
alkali to a mixed solution of cerous and ceric salts, the maximum 
intensity resulting when the ratio of cerous to ceric ion was 2 to 1. 

Hydrous Praseodymium Oxide.—The gelatinous mass of 
hydrous Pr.O3, precipitated from a praseodymium salt by alka- 
lies, is bright green in color and can be dried to a green powder 
which has, probably erroneously, been assumed to be a tri- 

hydrate.! If the hydrous oxide, the oxalate, or the nitrate of 
_ praseodymium is heated in air, a black powder is obtained, 
intermediate between PreO; and PrO2;? but the exact composi- 
tion depends on the substance calcined and the temperature of 
calcination.* In the presence of a small amount of CeOs, which 
appears to act as an oxygen carrier, the product approaches near 
the limit PrO:.4 It is probable that products of intermediate 
composition are not definite chemical individuals but are mix- 
tures representing intermediate stages in the oxidation of the 
lower oxide. In the present state of our knowledge, it is, of 
course, open to anyone to postulate an intermediate oxide, such 
as seems necessary to account for the color changes accompany- 
ing the oxidation of Ce.,.03;. By adding hydrogen peroxide to 
a praseodymium salt before precipitating, Braesner® claims to 
get Pr.O; - xH20. | 

Hydrous Scandium Oxide.—Alkalies and ammonia precipitate 
hydrous Sc2O03 as a white voluminous mass, insoluble in excess 
of precipitant. Like hydrous alumina, it is amorphous when 
first precipitated, but after ageing for some time, an x-radio- 
gram shows a transformation to a crystalline structure. When 
dried in the air at room temperature, it forms a hard horny mass 
which analyzes approximately for a trihydrate, Sc2O; - H.0;° 
but there is no definite evidence that such a hydrate exists. By 
dialyzing a solution of SnCl; to which ammonia is added short 


1 Cf. Damiens: Ann. chim., (9) 10, 181 (1918). 

2 WELsBACcH: Monatsh., 6, 477 (1885); Jones: Am. Chem. J., 20, 345 (1898) ; 
ScHOTTLANDER: Ber., 25, 569 (1892); Mryur: Z. anorg. Chem., 41, 97 (1904). 

3 ScHhELE: Ber., 32, 409 (1899). 

4 JAamR: Proc. Acad. Sci. Amsterdam, 16, 1095 (1914); Marc: Ber., 35, 
2382 (1902). 

5 Proc. Chem. Soc., 17, 66 (1901); cf. Mevixorr and Kuimenxo: J. Russ. 
Phys.-Chem. Soc., 33, 663, 739 (1901). 

6 Crooks: Phil. Trans., 209A, 15 (1909). 


260 THE HYDROUS OXIDES 


of precipitation, a hydrous sol results which sets to a jelly when 
treated with a suitable amount of electrolyte.! Under favor- 
able conditions this jelly is broken up by shaking, forming a 
limpid sol which will set again on being allowed to stand quietly.’ 

Hydrous Lanthanum Oxide.—The oxide La2O3 reacts with 
water with the evolution of heat like lime, giving a voluminous 
snow-white powder which has the formula La(OH)3; when dried 
at 100°. Although it dissolves slightly in water, the hydrous 
oxide thrown down by alkalies is almost as gelatinous as hydrous 
alumina; but the adsorption capacity of the latter for saccharose 
is appreciably greater.t The basic reaction of lanthanum hydrox- 
ide is comparable to that of ammonia;> hence, the gelatinous 
oxide absorbs CO, from the air and even the ignited oxide is 
readily soluble in acids. The basicity of the oxide would seem 
to preclude the formation of lanthanates, although Baskerville 
and Catlett® claim to have prepared complex compounds of this 
type by fusing lanthana with potassium hydroxide or by digest- 
ing the oxide with strong solutions of alkali. Undoubtedly, the 
products were hydrous lanthanum oxide with adsorbed alkali.” 

A transparent sol is obtained by peptizing the freshly formed 
hydrous oxide with a small amount of dilute hydrochloric acid.® 

Hydrous Neodymium Oxide.—The gelatinous oxide precipi- 
tated from a highly purified solution of a neodymium salt is 
blue and gives blue Nd,O3 on ignition. The blue color may be 
modified by the presence of impurities. By heating neodymium 
oxalate to a red heat in a stream of oxygen, Waegner® obtained 
a rose-colored product which gave a distinctly different reflection 
spectrum from Nd:O; and which appeared to be a higher oxide 
of the formula Nd,O;. By suitable choice of conditions, mixed 
spectra of Nd:O; and the so-called Nd4sO; were obtained. 
Similar observations were made on heating the hydrous oxide 


1 Boum and NicuassEen: Z. anorg. Chem., 132, 6 (1924). 

2 ScHALEK and Szecvanrt: Kolloid-Z., 33, 326 (1923). 

3 CLEVE: Bull. soc. chim., (2) 21, 196 (1874). 

4 EKuLER and Nitsson: Z. physiol. Chem., 181, 107 (1923). 

’ VESTERBERG: Z. anorg. Chem., 94, 371 (1916). 

6 J. Am. Chem. Soc., 26, 75 (1904). 

7 Cf. ZAMBONINI and CARosBi: Gazz. chim. ital., 54, 46, 53 (1924). 
8 Boum and NiciassEn: Z. anorg. Chem., 182, 6 (1924). 

9Z,. anorg. Chem., 42, 118 (1904). 


THE HYDROUS OXIDES OF THE RARE EARTHS 261 


and the anhydrous nitrate and carbonate. Joye and Garnier! 
claim that the different-colored products are not due to the oxy- 
gen content but to the degree of hydration of Nd,O3. Thus the 
hydrous oxide dried in air was taken to be Nd(OH);; on heating 
this to 320°, it has a formula corresponding to Nd2O3;- 1.5H.2O 
and gives a reflection spectrum corresponding to that of a 
similarly colored oxide described by Waegner; on further heating 
to 520°, the oxide has the composition Nd2O3;-H.2O and gives a 
reflection spectrum identical with Waegner’s Nd.O;. These 
data are interpreted to establish the existence of three hydrates 
of Nd.O3 and the non-existence of a higher oxide; but they are 
not conclusive. Thus, Garnier claims to get the same reflection 
spectrum by heating the hydrous oxide that Waegner does by 
heating what he says is an anhydrous salt, thereby precluding 
the formation of a hydrate. Of course, it may be argued that 
Waegner’s salts decomposed during dehydration, but this cannot 
be true, at least in the case of the carbonate which gives up all 
its hydrate water below 200°? and does not start to decompose 
until above 300°. Moreover, the view that the rose-colored 
product is a hydrate does not fit in with Waegner’s observa- 
tion that gentle heating in a current of hydrogen converts it 
into clear-blue Nd2,O3. Obviously, the whole problem should 
be reinvestigated. 

The blue gelatinous precipitate of the hydrous oxide is readily 
peptized by dilute HCl, forming a beautiful blue sol. 

Hydrous Samarium Oxide.—Gelatinous hydrous Sa,Q3 is 
almost white with a pale-yellow tinge which is not appreciably 
intensified on ignition to Sa2QO3. According to Cleve,‘ if the 
precipitation with ammonia is carried out in the presence of 
hydrogen peroxide, a hydrous oxide of the formula Sa4Op» - tH2.O 
results which is similar in appearance to hydrous S8a2Q3. 


Tur Hyprovus OXIDES OF THE TERBIUM FAMILY 


The hydrous oxides of europium, gadolinium, and terbium are 
obtained in the same way as the corresponding compounds of 

1Compt. rend., 134, 510 (1912); GarnimrR: Arch. sct. phys. nat., (6) 40, 
98, 199 (1915). 

2 Preiss and Rainer: Z. anorg. Chem., 131, 287 (1923). 


3’ Boum and NiciassEen: Z. anorg. Chem,, 132, 6 (1923), 
4 Bull. soc. chim., (2) 48, 53 (1885). 


262 THE HYDROUS OXIDES 


the cerium family, by the action of alkali or ammonia on solutions 
of their salts. When freshly prepared, the gelatinous oxides 
rapidly absorb carbon dioxide when exposed to the air. Anhy- 
drous Gd2,03 and Tb2O3 are white solids, while Ku2O3 possesses 
a reddish-yellow tinge.! All of the oxides are soluble in acids; 
but Gd.O; dissolves very slowly at the start, the velocity increas- 
ing as the action proceeds.2, When terbium oxalate is ignited, it 
gives a dark-brown peroxide which approaches the composition 
required for TbOs. If a mixture of air and coal gas is passed 
over TbO:, or a mixture of Gd2O3 and TbO2 heated almost to 
redness, the whole mass immediately becomes incandescent, 
and the gas often takes fire.® 


THE Hyprovus OXIDES OF THE YTTRIUM FAMILY 


Dysprosium, holmium, erbium, thulium, yttrium, ytterbium, 
and lutecium all form highly gelatinous oxides when thrown down 
from their salt solutions with ammonia. Like hydrous alumina, 
the gels of Er.03 and Y203 become microcrystalline on standing,‘ 
and it is probable that the other oxides behave similarly. Ho2O3 
has a pale-yellow color; Er.Os is rose red; Tm2O3 is white with a 
greenish tinge; and Dy203, Y2O3, Yb2O3:, and Lu2O; are white. 
Hydrous peroxides of yttrium Y,4O,- #H.O and of erbium ErQ, :- 
«H,O are formed by adding hydrogen peroxide and ammonia 
to solutions of their respective salts.® 

On account of the gelatinous character of the precipitated 
oxides, it is probable that all of them will form sols; but so far 
only two have been described. Bohm and Niclassen® dialyzed a 
solution of erbium nitrate to which ammonia was added short of 
precipitation. This sol set to a jelly on adding a suitable amount 
of precipitating electrolyte. Miller’ peptized the hydrous oxide 
of yttrium with dilute hydrochloric acid, aluminum chloride, 
and ferric chloride; and Szilard® employed thorium acetate. 

1PRANDTL: Ber., 55B, 692 (1922). 

2 BENEDICKs: Z. anorg. Chem., 22, 392 (1900). 

3 BissEL and JamEsS: J. Am. Chem. Soc., 38, 873 (1916). 

4 Boum and NICLASSEN: Z. anorg. Chem., 182, 1 (1924). 

5 CLeve: Bull. soc. chim., (2) 21, 196 (1874). 

6 Z. anorg. Chem., 182, 6 (1924). 

7Z. anorg. Chem., 57, 314 (1908). 

8 J. chim. phys., 5, 488, 636 (1907). 


CHAPTER XI 
THE HYDROUS OXIDES OF THE FIFTH GROUP 


The elements of the fifth group which form hydrous oxides are 
vanadium, columbium, tantalum, antimony, and bismuth. These 
will be taken up in the order named. 


Hyprovus VANADIUM PENTOXIDE 


The addition of a mineral acid to a concentrated solution of an 
alkali or alkaline earth vanadate throws down V.O; as a red- 
brown amorphous hydrous mass, closely resembling hydrous ferric 
oxide. A similar precipitate results from the hydrolysis of vana- 
dium oxychloride. The gel is made up of very fine particles 
which cannot be washed free from the mother liquor without 
undergoing peptization. By drying in the air, von Hauer? real- 
ized a composition approaching that of a dihydrate which was 
taken to be pyrovanadic acid, H4V.O,;, analogous to the corre- 
sponding phosphorus compound. Continuing the drying over 
sulfuric acid until another molecule of water is lost, gives the 
correct formula for metavanadic acid, HVO;.2 The exact inves- 
tigations of Dullberg* show, however, that the red-brown gel is 
not an acid but is hydrous vanadium pentoxide whose water 
content depends on the condition of drying. The so-called pyro- 
and metavanadic acids not only do not occur as solids but are 
incapable of existing in solution, although both pyro- and 
metavana dates are known. The stronger hexavanadic acid, 
H4V,O17 or 6V20; - 2H.O, does exist in dilute solution, but the 
solid acid is unknown. 


1 MorssaNn: Bull. soc. chim., (3) 15, 1278 (1896). 
2 J. prakt. Chem., 80, 324 (1860). 
3 FRITZSCHE: J. prakt. Chem., 58, 93 (1851); Manassrm: Liebig’s Ann. 
Chem., 240, 23 (1887). 
4Z, physik, Chem., 45, 129 (1908). 
263 


264 THE HYDROUS OXIDES 


VANADIUM PENTOXIDE SOLS 


Biltz! treated ammonium vanadate with a dilute solution of 
hydrochloric acid, obtaining vanadium pentoxide as a brownish- 
red powder. After thorough washing to remove excess elec- — 
trolyte, the oxide peptizes completely in water, giving a clear 
reddish-yellow sol. The colloidal particles are negatively 
charged and are highly hydrous. Addition of ammonium 
chloride to a concentrated sol causes it to set to a jelly; while a 
dilute sol is precipitated as reddish-yellow highly gelatinous 
flocs that settle very slowly. If the washed oxide is dried 
before being peptized, the particles in the sol are larger and are 
precipitated in less voluminous floes which settle more rapidly. 
Wegelin? prepared vanadium pentoxide by hydrolysis of a boiling 
solution of VOCI;. This was peptized by washing; but the 
particles are larger and less hydrous than those in the Biltz sol. 
When treated with electrolytes, the particles agglomerate into 
dense clumps that settle out rapidly. The precipitate from a 
boiled Biltz sol is likewise much denser and darker than from an 
unboiled sol. | 

Miiller* obtained a sol by triturating the granular mass pro- 
duced by sudden cooling of molten vanadium pentoxide either 
by plunging the containing vessel of platinum into cold water or 
by pouring the melt into cold water. Sols formed in this way 
are reddish brown in color. The precipitation by ammonium 
chloride is reversible; but the dense brown residue obtained by 
evaporating the sol to dryness on the water bath is not repeptized 
by water, whereas the looser yellow mass resulting from evapora- 
tion of the Biltz sol is easily repeptized. 

Freundlich and Leonhardt® peptized an amorphous, ocher- 
yellow oxide obtained by gentle ignition of ammonium vanadate. 
This takes up water from air saturated with moisture, the color 
becoming reddish yellow. The sol formed by triturating with 
a little water, followed by shaking with an excess of water, is 


1 Ber., 37, 1098 (1904). 

2 Rong. Z., 11, 25:(1912). 

8 Himeeneice and LEONHARDT: Rollsidehern Bethefte, T, 193 (1915). 

4 Kolloid-Z., 8, 302 (1911). 

6 Kouchner Bethefte, 7, 187 (1915); cf. Dirrn: Compt. rend., 101, 699 
(1885). 


THE HYDROUS OXIDES OF THE FIFTH GROUP 265 


orange yellow in color; but on standing for several days with 
occasional shaking, it changes to a yellowish red. 

The esters of orthovanadic acid are readily hydrolyzed by 
water, and Prandtl and Hess! took advantage of this reaction to 
prepare “‘electrolyte-free’”? vanadium pentoxide sols. For this 
purpose, the tertiary butyl ester is particularly satisfactory, 
both because it is a stable salt and because the hydrolysis product, 
tertiary butyl alcohol, can be removed from the sol almost com- 
pletely by boiling. The sols are orange when first prepared, but 
are changed to yellowish red by heating. 

While two modifications of vanadium pentoxide have been 
described—a yellow amorphous form and a red crystalline form— 
the observations recorded in the preceding experiments indicate 
that the color is influenced to a marked extent by the degree of 
dispersion. ‘The most highly dispersed oxide appears yellow, 
the color changing to reddish brown as the particles become 
larger and denser. If this view is correct, the reddish crystalline 
oxide should be yellow if sufficiently finely divided. As a matter 
of fact, Wegelin? prepared a canary-yellow sol by prolonged tri- 
turation, in an agate mortar, of red-brown crystals of vanadium 
pentoxide obtained by allowing the molten oxide to cool slowly. 
If this sol is coagulated by the addition of a small amount of 
sodium chloride, the yellow precipitate shows little change of 
color on keeping; but if a larger amount of sodium chloride is 
used in the coagulation, the resulting precipitate changes its 
color in the course of a few days from yellow to reddish brown. 
This change in color is due to growth of the particles, a process 
which Freundlich*® has found to increase rapidly with increasing 
concentration of electrolyte in contact with a precipitate. 

In vanadium pentoxide sols there is always a small amount of 
the oxide in molecular solution. This portion, yellow in color, 
passed through a dialyzing membrane and is not thrown down by 
electrolytes. As the sol is slightly acid, the yellow solution is a 
vanadic acid, possibly hexavanadic, H4V6QO17,*+ which yields a 
yellow anion, [HV.Q.7]’’. The presence of a tervalent anion 


1Z. anorg. Chem., 82, 116 (1913); cf. RiepeL: Pharm. J., 92, 648 (1914). 
2 Kolloid-Z., 14, 65 (1914). 

3 FREUNDLICH and Haske: Z. physik. Chem., 89, 446 (1915). 

4 DuLLBERG: Z. physik. Chem., 45, 175 (1903). 


266 THE HYDROUS OXIDES 


which is likely to be strongly adsorbed accounts for the negative 
charge on the colloidal particles.1 As might be expected, the 
solubility determinations of different investigators show wide 
variations owing to the influence of particle size on solubility. 
Moreover, the usual measurements made on the supernatant 
solution after agglomeration of the sol are necessarily wrong, 
since they fail to take into account the amount adsorbed by the 
hydrous particles during precipitation. 

Optical Properties.—Probably the most interesting property 
of vanadium pentoxide sol is its double refraction on stirring, a 
phenomenon first observed by Freundlich and his pupils.? If 
stirred with a glass rod and viewed in reflected light, an aged sol 
appears to be filled with yellow glittering streaks as if there were 
fine crystals suspended in it. In transmitted light, the sol 
remains clear, but dark streaks can be observed. Viewed between 
crossed nicols, the field remains dark as long as the sol is not dis- 
turbed; but stirring causes the field to become bright at once. 
By allowing the sol to flow between crossed nicols in convergent 
light parallel to the line connecting the nicols, an image is ob- 
tained of a crossed axis with concentric rings. Observed with a 
quarter-wave mica plate, the flowing sol behaves like a positive 
uniaxial crystal. Freundlich pictures the sol at rest as made up 
of elongated particles possessing the usual unordered Brownian 
movement which can give no double refraction. The setting up 
of directed motion causes the sol to lose its isotropic nature and 
to become double refracting. A section cut from the sol may be 
looked upon as having a space lattice somewhat similar to a plate 
from an optically monoaxial crystal, the long axis of the sol 
particles coinciding in direction with the optical axis. 

If the sol is rotated between two cylindrical walls and viewed 
between crossed nicols, four minima of brightness are seen, giving 
the appearance of a dark cross,* the arms of which form an angle 
with the direction of polarization. The angle is independent 


1 OSTERMAN: Wissench. u. Ind., 7, 17 (1922); Chem. Zentr., I, 396 (1923); 
cf., however, Dumanskt: Kolloid-Z., 38, 147 (1923). 

2 FREUNDLICH and LronnuarpDT: Kolloidchem. Bethefte, 7, 207 (1915); 
DresseLHorstT and Freunpiicu: Physik. Z., 16, 422 (1915); Freunp.icu: 
Z. Elektrochem., 22, 27 (1916). 

3 ZOCHER: Z. physik. Chem., 98, 293 (1921). 


THE HYDROUS OXIDES OF THE FIFTH GROUP 267 


of the concentration of sol but increases rapidly with increasing 
age of sol-and decreases with rise of temperature. In a slowly 
moving fresh sol, the angle has the value of 45°, and in a rapidly 
moving aged sol, it approaches 90°. This behavior of the 
so-called vortex cross has been explained by Freundlich! in terms 
of the elasticity of the sol.? In fresh, slowly moving sols the 
elastic deformation of the sol elements is small; and so the -sol 
behaves like a rigid body and the cross-angle is 45°. In an aged 
rapidly moving sol, the angle is close to 90°. From this point 
of view, the cross-angle is identical with the angle of maximum 
deformation; and the direction of maximum deformation cor- 
responds with the direction of the velocity gradient. Hence, 
the colloidal particles do not arrange themselves along the 
line of motion because of friction between adjacent liquid layers 
of different velocities, but place themselves in the direction of 
maximum deformation. Only in an aged sol moving with high 
velocity does the direction practically coincide with the direction 
of flow, giving a vortex cross with 90° angles. 

The double refraction in an aged sol is so strong that it can be 
demonstrated by allowing the sol to flow through a prismatic 
trough with a triangular cross-section and using this as a prism 
to decompose spectrum lines. In this way, the red hydrogen 
line is resolved into two oppositely polarized lines. The more 
strongly refracted ray vibrates parallel to the direction of flow 
of the sol, and in accordance with Babinet’s rule, this extraordi- 
nary ray is more strongly absorbed than the other.‘ 

As already noted, the double refraction is not observed in a 
freshly prepared vanadium pentoxide sol. Freundlich®investi- 
gated quantitatively the influence of age of sol on its double 
refraction and found the velocity of ageing at constant streaming 
velocity and temperature, to be given by the equation dA/dt = kA 


1 FREUNDLICH, STAPELFELDT, and ZocHEeR: Z. physik. Chem., 114, 161, 
190 (1924); cf., however, MortsmirH and Lanemutir: Phys. Rev., (2) 20, 
95 (1922). 

2 FREUNDLICH and Sgrirritz: Z. physik. Chem., 104, 233 (1923). 

3 Scowestorr: J. phys., (3) 1, 49 (1892). 

4Cf. also HumpHrey: Proc. Phys. Soc. London, 35, 217 (1923). 

’ FREUNDLICH, STAPELFELDT, and ZocHER: Z. physik. Chem., 114, 161 
(1924); cf. GessNER: Kolloidchem, Bethefte, 19, 283 (1924). 


268 THE HYDROUS OXIDES 


(Ac — A)?, where A is the double refraction. The magnitude 
of the velocity of ageing is very sensitive to the action of impuri- 
ties which may have either a stabilizing or peptizing action on 
the sol. With rising temperature the anisotropy decreases in a 
linear fashion. The double refraction of the sol corresponds 
approximately to that of the vanadium pentoxide content. 

Examination of an aged sol with the cardioid ultramicroscope! 
reveals rod-like structures whose length is approximately thirty 
times the diameter. In a slit ultramicroscope, their axis deviates 
by less than 30° from a line perpendiculr to the axis of the illum- 
inating beam. Reinders? believes the appearance of birefring- 
ence on ageing is due to the formation of ultramicroscopic needles, 
since he succeeded in demonstrating a similar birefringence in 
sols of mercurous chloride and lead iodide which ordinarily 
form microscopic crystals. Later, Zocher? established the cry- 
stalline character of the particles in an aged vanadium pentoxide 
sol by means of x-radiograms. The interference lines are broad, 
indicating the very small size of the crystals in the sol; but the 
arrangement of the lines is the same as observed with crystals 
obtained by cooling the molten pentoxide. The effect of ageing 
on the dielectric constant of the sol* indicates that the growth of 
rod-shaped particles during the ageing process is not an ordinary 
case of crystallization.> Freundlich® attributes the appearance 
of double refraction, on adding electrolytes to a benzopurpurin 
sol, to the development of longer particles by ordered coagulation 
and not to the growth of needle crystals. Just why we should 
get ordered coagulation into rod-shaped particles only in certain 
cases 1s not obvious. 

The phenomenon of streaming double refraction is not confined 
to sols of vanadium pentoxide, but has been observed with an 
aged ferric oxide sol, soap solutions, clay suspensions, silver 
cyanate, and a number of dyes such as benzopurpurin, alizarin, 


1Kruyt: Proc. akad. Wetenschappen, 18, 1625 (1916); Kolloid-Z., 19, 
161 (1916). 

2 Proc. akad. Wetenschappen, 19, 189 (1916). 

3Z. physik. Chem., 98, 312 (1921). 

4 HERRERA: Kolloid-Z., 31, 59 (1922); 32, 373 (1928). 

5 SzEGVARI and WIGNER: Kolloid-Z., 33, 218 (1923). 

6 FREUNDLICH, SCHUSTER, and ZocHER: Z. physik. Chem., 106, 119 (1923). 


THE HYDROUS OXIDES OF THE FIFTH GROUP 269 


and aniline blue.! In most of these cases, the double refraction 
appears to be due in large méasure to the anisotropic nature of 
the particles themselves rather than to the lattice-like arrange- 
ment of rod-shaped isotropic particles. On the other hand, with 
tungsten trioxide sol, the form and size of the particles is the 
important factor in changing the nature of the Tyndall lght, 
while the anisotropy of the particles is negligible. 

Attention has been called to the greater adsorption of the 
extraordinary ray than of the ordinary ray by a streaming vana- 
dium pentoxide sol. This gives rise to dichroism which may be 
termed streaming dichroism. When the intensity of transmitted 
polarized light whose electric victor vibrates perpendicular to 
the direction of flow of the sol is decreased by allowing the sol to 
flow, the light appears redder. As a matter of fact, the spectrum 
of the flowing sol extends from 710 to 582uu; while at rest, it 
extends only to 558uun. With parallel electric victor, the light 
appears yellower when the sol flows, the spectrum extending 
only to 542uy.? | 

Coagulation of Sol. Jellies—Vanadium pentoxide is quite 
sensitive to the action of most electrolytes as evidenced by the 
relatively low concentration necessary to cause clouding of the 
sol within 5 minutes, the so-called ‘‘clouding value,’’ Table 
XXIV.* With but few exceptions the clouding value is lower, the 
greater the valence of the precipitating ion; but ions of the same 
valence show the usual large variations from a constant value. 
The precipitated gel is very readily repeptized by washing, pro- 
vided the precipitating ions are not too strongly adsorbed. 

A sol containing 5 grams V.O; per liter sets to a stiff jelly when 
coagulated by suitable concentrations of electrolytes. This 
might be expected in view of the strong tendency of the oxide in 
mass to adsorb water and swell. A fresh sol contains smaller and 
more hydrous primary particles and gives a more gelatinous 
precipitate than an aged sol. It is probable that the relatively 
rapid coagulation in the presence of electrolytes gives fibrils just 
as the slow agglomeration during a long interval gives the 


1 ZocunEr: Z. physik. Chem., 98, 293 (1921); FreunpLicH, SCHUSTER, and 
ZocHER: [bid., 105, 119 (1923). 

2 DigssELHORST and FrREuNDLICH: Physik. Z., 16, 419 (1925). 

3 FREUNDLICH and LEONHARDT: Kolloidchem, Bethefte, 7, 195 (1915). 


270 THE HYDROUS OXIDES 


' TasBLE XXIV.—CLoupING VALUE OF VANADIUM PENTOXIDE SOLS 





Clouding Clouding 
Electrolyte value, ee Electrolyte ra aN 
equivalents equivalents 
per liter per liter 
Ti Clase Sy er ek eas 130.0 Sr(NOs)> ois" lhe arate ee Rea 0.562 
INS ts eee eae: © 50.0 BaCl:. &. 227 eee 0.46 
NH,Cl.. 2040 WnSOiws | i, ee 1.68 
Oe a eas 17.0 VO8S0O4... 5.4 7 eee 1.26 
id ® Ea Sete RO TE Pb(C,H;03):. 3s 0.62 
AGING) See ets ae cares Onl CuS0¢ oS eee 0.78 
T1804 SWetenatne: Sushi ctis Remain stats 0.51 HgCl, a pau leteeedhaeeaelas ia oom 0: 726 
Guanadine nitrate...... 4.0 Ce(NOs)6§:..-<c) iiea a eee es 
Strychnine nitrate...... 0.17 Als (SQi) sce ee 0.00168 
Meg(NO;)s.....5 eg. 0 UN: Th(NOs)4 stein pened eee 0.0168 





rod-shaped birefringent particles. Thus vanadium pentoxide 
appears to possess in high degree the characteristics necessary 
for jelly formation, and it would be interesting to know the 
minimum dilution of sol that could be made to form a typical 
jelly by coagulation. 

Hydrous vanadium pentoxide prepared under suitable condi- 
tions forms a fine yellow pigment termed vanadium bronze which 
is employed as a substitute for gold bronze. It is obtained in 
the form of brilliant scales of a golden or orange color by boiling 
aqueous sulfurous acid with copper vanadate.' It is also pre- 
pared by adding a solution of ammonium vanadate to one of 
copper sulfate containing an excess of ammonium chloride, until 
a permanent precipitate forms, followed by heating gently to 
75°. The slower the precipitation, the finer is the color of the 
bronze. 

The hydrous oxide finds some application as a mordant in the 
dyeing of cotton and especially in fixing aniline on silk. The 
anhydrous oxide is an efficient catalyst for the oxidation of cer- 
tain organic compounds, such as the oxidation of sugar by nitric 
acid and the oxidation of aleohol by atmospheric oxygen.? By 


1 GERLAND: Bull. soc. chim., (2) 19, 501 (1873); Ber., 10, 1515 (1877). 
2 Moser and LInDENBAUM: J. prakt. Chem., (2) 75, 146 (1907). 


THE HYDROUS OXIDES OF THE FIFTH GROUP 271 


heating ammonium vanadate with resin or linseed oil, the pen- 
toxide is obtained in suitable condition to serve as a dryer for lin- 
seed oil. This drier produces a smooth tough film; but there is 
some darkening of the oil.! 


LOWER OXIDES OF VANADIUM 


In addition to the pentoxide, vanadium forms a suboxide, 
VO; a monoxide, VO; a sesquioxide, V2O3; and a dioxide, VO» 
or V2O,. The last two form hydrous oxides. 

Vanadium Dioxide.—When a solution of vanadyl sulfate or 
chloride is treated cautiously with a cold solution of sodium 
carbonate, hydrous vanadium dioxide? comes down as a greyish- 
white precipitate soluble in excess of precipitant. The oxide 
takes up water from the air and must be washed in an inert 
atmosphere. Dried over sulfuric acid, it is a black amorphous 
mass exhibiting a glassy fracture when broken. It happens to 
analyze approximately for V2O,4- 7H2O when dried over sulfuric 
acid at room temperature and for V20O,-: 38H2O when heated at 
100°. These formulas are, of course, purely accidental. Gain# 
claims to have prepared a pale-red crystalline hydrate V.O, :- 
2H,20 by boiling a solution of the dioxide in sulfuric acid. When 
kept out of contact with the moisture of the air, it loses its red 
color, becoming olive green. The two compounds are said to 
be isomers. The dioxide is insoluble in water but dissolves in 
both acids and alkalies, forming vanadyl salts* and vanadites or 
hypovanadates,°* respectively. 

Vanadium Sesquioxide—Hydrous vanadium  sesquioxide 
comes down as a dirty-green flocculent precipitate when an aque- 
ous solution of vanadium trichloride is treated with ammonia.® 
It is extremely unstable in the air, oxidizing to the dioxide very 
quickly. The hydrous oxide precipitated and washed in an 
inert atmosphere has been used as the starting point in the 


1 RuopveEs and CuHen: J. Ind. Eng. Chem., 14, 222 (1922). 

2 Crow: J. Chem. Soc., 30, 454 (1876). 

3 Compt. rend., 148, 823, 1154 (1906); 146, 403 (1907). 

4GuyarpD: Bull. soc. chim., (2) 25, 350 (1876). 

5 Koppri and GoLpMANN: Z. anorg. Chem., 36, 281 (1998). 

®Lockm and Epwarps: Am. Chem. J., 20, 594 (1898); Prccin1 and 
Brizzi: Z. anorg. Chem., 19, 394 (1899). 


272 THE HY DROUS OXIDES 


preparation of a number of salts of trivalent vanadium. It 
functions as a basic oxide only. 


Hyprous COoOLUMBIUM PENTOXIDE 


The hydrolysis of CbCl; or CbOCl; yields an amorphous white 
gelatinous mass of hydrous columbium pentoxide. It is also 
formed by the action of sulfuric acid on a solution of alkali 
columbate. When dried at room temperature, it is a horny 
mass; at 100°, it is a white powder which retains varying amounts 
of water depending on the history of the sample. The oxide is 
insoluble in water; hence, it must be regarded as a hydrous 
oxide and not as columbie acid. 

Like vanadium pentoxide, the hydrous mass cannot be washed 
free from the mother liquor without undergoing peptization. 
It dissolves but slightly in hydrochloric acid, but the residue 
obtained after boiling with excess acid is easily peptized by 
water, giving a sol from which practically all the hydrochloric 
acid can be removed by dialysis. The sol gradually clouds up on 
standing and is coagulated completely by electrolytes. The 
oxide thrown down from the hydrochloric acid sol with ammonia 
is an aged oxide corresponding to the so-called metatitanic acid. 
It is said to be a definite hydrate, 3Cb20; - 4H.O,? but this is 
improbable. 

Hauser and Lewite® prepared negative sols of both columbium 
and tantalum pentoxide by fusing the respective oxides with 
alkali in a silver crucible, dissolving the melt in water, and dialyz- 
ing for 10 to 12 days. Both sols are quite stable, even when 
heated, but are precipitated by all strong electrolytes except 
bases which stabilize them owing to preferential adsorption of 
hydroxyl ion. The two sols differ in their behavior toward 
carbon dioxide. If the gas is conducted into the tantalum 
pentoxide sol, complete coagulation takes place in a short time, 
whereas a sol of columbium pentoxide does not coagulate for a 
day under the same conditions. The Weiss-Landicker* method 


1 WOHLER: Pogg. Ann., 48, 93 (1839); Marianac: Ann. chim. phys., (4) 
13, 20 (1868). 

2 SANTESSON: Bull. soc. chim., (2) 24, 52 (1875). 

3Z. angew. Chem., 20, 100 (1912). 

4Z. anorg. Chem., 64, 65 (1909). 


’ THE HYDROUS OXIDES OF THE FIFTH GROUP 273 


of separating columbium from tantalum is based on this difference 
in behavior of the sols. The method is of value only for the 
qualitative separation and detection of the elements. 


Hyprovus TANTALUM PENTOXIDE 


Gelatinous tantalum pentoxide is obtained by the hydrolysis 
of tantalum pentachloride with an excess of water and by treat- 
ing sodium tantalate with sulfurous or nitric acid. The gel 
exhibits the glow phenomenon when ignited, unless it has been 
aged by washing with hot water. A granular hydrous oxide is 
formed by fusing the anhydrous pentoxide with potassium bisul- 
fate and boiling the resulting mass with water. The compounds 
prepared in different ways show a variable water content when 
dried at 100°. Formulas corresponding to hydrates have been 
suggested! for the dried oxide, but recent observations of Jander 
and Schulz? fail to establish their identity. The hydrous oxides 
investigated by Jander and Schulz were prepared by adding an 
excess of dilute nitric acid drop by drop to a solution of sodium 
tantalate of the composition 7Na2O-5Ta:,0;:40H2O, at 0 and 
100°. The voluminous amorphous precipitates were filtered on a 
membrane filter,* washed with large amounts of water, and dried 
in vacuum over sulfuric acid. The vapor-tension isotherms were 
determined in the vacuum apparatus described by Zsigmondy.* 
The results recorded in Fig. 17 show a continuous variation in 
the water content of the oxide with changing vapor pressure 
of the surroundings, thereby rendering the existence of hydrates 
improbable. The rehydration and redrying curves are similar 
qualitatively to those observed by van Bemmelen with the gels 
of stannic oxide, silica, etc. The hysteresis cycle of the 100° 
oxide is smaller than that of the 0° oxide and is displaced more 
toward the side of lower water content. The optical phenomena 


1 Rose: Pogg. Ann., 100, 417 (1857); 106, 141 (1839); RaAMMELSBERG: 
Ibid., 136, 177, 325 (1869); Hermann: J. praki. Chem., (2) 5, 66 (1872); (1) 
70, 195 (1857). 

2Z. anorg. Chem., 144, 225 (1925). 

3 ZsIGMONDY and BAcHMANN: Z. anorg. Chem., 103, 119 (1918); Zst1aMonDyY 
and JANDER: Z. anal. Chem., 58, 241 (1919); 63, 673 (1928). 

47ZSIGMONDY, BACHMANN, and STEVENSON: Z. anorg. Chem., 75, 189 
(1912). 


274 THE HYDROUS OXIDES 


so characteristic of the hydration and dehydration of silica were 
lacking in the tantalum pentoxide gels; both remained white and 
chalky under all conditions. 

Sols of hydrous tantalum pentoxide are obtained in the same 
way as the corresponding columbium pentoxide sols already 
described. The hydrous oxide is insoluble in water but is slightly 
acidic in character, dissolving in alkalies with the formation of 
tantalate. 


Vapor Pressure, millimeters of mereury 





Fic. 17.—Vapor pressure diagram of tantalum pentoxide. 


Tur Hyprovus Oxiprs or ANTIMONY 


Antimony Pentoxide.—On account of the position of antimony 
in the periodic system of the elements, antimony pentoxide is 
usually supposed to form ortho-, pyro-, and metaantimonic 
acids: H;SbO, or Sb20O;:3H2O; H4iSbeO7 or Sb20;-2H2O; and 
HSbO; or Sb2O;- H20,! corresponding to the acids of phosphorus. 
In addition to these, hydrates containing 4, 4.5, 5, and 6 mols of 
water per mol of pentoxide have been described.” ‘The oxide is 


1See Fremy: J. prakt. Chem., 48, 293; 45, 209 (1848); Hurrrner: Pogg. 
Ann., 86, 419 (1852); GruTHER: J. prakt. Chem., (2) 4, 438 (1871); Dav- 
BRAWA: Liebig’s Ann. Chem., 186, 110 (1877); Conrap: Chem. News, 40, 197 
(1879); SENDERENS: Bull. soc. chim., (8) 21, 47 (1899); Dexacrorx: Jbid., 
(3) 21, 1049 (1899). 

2 SENDERENS: Bull. soc. chim., (3) 21, 47 (1899). 


THE HYDROUS OXIDES OF THE FIFTH GROUP 2795 


thrown down in a flocculent hydrous condition by hydrolyzing 
antimony pentachloride in water or antimony trichloride in 
nitric acid solution; and by decomposition of a solution of potas- 
sium pyroantimonate with acids. A survey of the procedures 
that must be followed to get a composition approximating a 
hydrate suggests that the so-called ortho-, pyro-, and meta- 
antimonic acids are, in reality, hydrous antimony pentoxides 
dried under such conditions that the composition approaches 
that of the corresponding acids of phosphorus. This conclusion 
has been confirmed by the observations of Jander and Simon! 
on oxides prepared in a variety of ways. In Table XXV is 
given the composition of oxides precipitated at different tempera- 


TABLE XX V.—CompPposITION oF Hyprous ANTIMONY PENTOXIDE 


Mols of water per mol of Sb2O; 





Condition of drying 








0° oxide | 60° oxide | 100° oxide 
UO Os oS) 30.56 9.97 7.91 
Over concentrated H2SO.............. 3.68 eS We 0.60 
US AGUS Ae co a ee ar 2.43 1.02 0.45 


tures and dried under varying conditions. It is quite obvious 
that the composition is determined by the conditions of precipi- 
tation and the method of drying, and that a composition cor- 
responding to a hydrate is purely accidental. This is further 
emphasized by the unbroken character of the dehydration- 
velocity-composition curves and the vapor-pressure-composition 
curves of various preparations. The vapor-pressure isotherms 
are very similar to those shown in Fig. 17 for hydrous tantalum 
pentoxide. All the oxides show the characteristic hysteresis 
region. The 100° preparation exhibits optical phenomena in 
this region similar to those observed by van Bemmelen. with 
hydrous stannic oxide. At the beginning of the rehydration the 
oxide is a transparent glassy mass; as more water is taken up, 
it becomes cloudy and the color changes to brown; on complete 
hydration, the gel is clear and colorless once more. 

Like hydrous stannic oxides, the different antimony pentoxides 
adsorb alkali salts and phosphoric acid, giving typical adsorption 

1 Kolloid-Z., 23, 122 (1918); Z. anorg. Chem., 127, 68 (1923). 


276 | THE HYDROUS OXIDES 


isotherms. As would be expected, the loose finely divided 0° 
oxide has the highest adsorption capacity, and the dense granular 
100° oxide has the lowest adsorption capacity for salts as well 
as water. The hydrous oxides adsorb alkalies from dilute solu- 
tions, giving amorphous masses of indefinite composition that 
have been mistaken for definite antimonates. More concen- 
trated solutions dissolve the oxides, and from these solutions 
definite crystalline antimonates are obtained. 

Alcogels of antimony pentoxide are formed by treating the 
hydrogels with gradually increasing amounts of alcohol. If 
maintained over glycerol, dealcoholation curves are obtained 
similar to those of dehydration. 

Hydrous oxides freshly prepared by hydrolysis of antimony 
pentachloride are peptized by thorough washing, forming instable 
sols from which an aged oxide gradually separates on standing or 
heating. Delacroix! and Senderens? mistook these sols for 
molecular solutions, and so reported erroneous values for the 
solubility of Sb.Os in water. 

Antimony Trioxide.——Like phosphorus trioxide, antimony 
trioxide is supposed to form ortho-, pyro-, and metaantimonous 
acids, H3SbO3 or SbeO3:3H20, HaSbeOs or SbeO3-3H2O, and 
HSbO:z or Sb203: H20, respectively. Clark and Stallo*® claim to 
have prepared the trihydrate or ortho acid by the action of sul- 
furic acid on barium antimony] tartrate. The addition of mineral 
acids to tartar emetic yields a hydrous product containing vary- 
ing amounts of adsorbed tartaric acid and precipitating agent 
which are difficult to remove by washing.* Using alkaline car- 
bonates, acetates, phosphates, and tungstates as precipitants, 
Long? finds the composition to be approximately Sb203: 0.5H20. 
Schaffner® reports the formation of a dihydrate, pyroanti- 
monous acid, by treating an alkali solution of arsenious sulfide 
with copper sulfate until all the sulfur is removed, and then 


1 Bull. soc. chim., (3) 21, 1049 (1899); 25, 288 (1901). 

2 Bull. soc. chim., (3) 21, 47 (1899). 

3 Ber., 18, 1792 (1880). 

4 Gunz: Compt. rend., 102, 1472 (1886). 

6 J. Am. Chem. Soc., 17, 87 (1891). 

6 Liebig’s Ann. Ghee 51, 168 (1844); cf., however, SERONO: Gazz. chim, 
ital., 24, 274 (1894). 


THE HYDROUS OXIDES OF THE FIFTH GROUP 277 


acidifying. Lea and Wood! found that the method of Clark and 
Stallo does not yield orthoantimonous acid but a hydrous oxide 
of varying composition, depending on how it is washed and 
dried. Long’s method likewise yielded products of varying 
water content, depending on the temperature of precipitation. 
The oxide thrown down with mineral acids contains more 
adsorbed impurities and retains its adsorbed water more strongly 
than the compound precipitated with alkalies and alkali carbon- 
ates. By treating a solution of antimony trichloride in hydro- 
chloric acid with ammonia, a very finely divided hydrous mass 
is first formed which goes over to the anhydrous oxide on warming. 
In this respect, its behavior reminds one of hydrous cupric oxide. 

Antimony trioxide is insoluble in water but is amphoteric in 
character, giving antimonous salts with acids and antimonate 
with alkalies. Heated in air, the trioxide takes on more oxygen, 
giving the tetraoxide, Sb2O.. 


Hyprovus OxipEs or BISMUTH 


Bismuth Trioxide.——The most common oxide of bismuth is 
the trioxide, BizO3. This is thrown down quantitatively in the 
cold by alkalies and ammonia as a white flocculent mass, errone- 
ously assumed to be Bi(OH); or BizO3-3H.O. If precipitated 
from chloride or nitrate solution, the hydrous oxide is usually 
contaminated by oxychloride or nitrate; but if the solution of 
the oxide in alkali in the presence of glycerin? is acidified, the 
compound comes down in a highly gelatinous form free from 
basic salt.* A pure preparation is said to result also from pouring 
a solution of bismuth nitrate into dilute alkali* and by precipi- 
tating an acid solution of bismuth nitrate with concentrated 
ammonia.> The hydrous mass has a composition approximating 
Bi(OH)3; only when dried in air. When dried at 100°, it is usu- 
ally assumed to form metahydroxide, BiOOH or Bi,0;-2H,0,° 

1J. Chem. Soc., 123, 259 (1923). 

2 Lowe: Z. anal. Chem., 22, 498 (1883). 

3 THIBAULT: J. Pharm., (6) 12, 559 (1900). 

4 PripEAux and Hewis: J. Soc. Chem. Ind., 41, 167 (1922). 

5 Motes and Portituo: Chem. Zentr., I, 33 (1924). 

6 ArpPPE: Pogg. Ann., 64, 237 (1845); Rupp: Z. anal. Chem., 42, 732 (1903); 
Moser: Z. anorg. Chem., 61, 379 (1909); Pripkaux and Hewis: J. Soc. 
Chem. Ind., 41, 167 (1922). 


278 THE HYDROUS OXIDES 


but this could not be confirmed.! Apparently the so-called ortho- 
and metahydroxides merely represent stages in the continuous 
dehydration of a hydrous gel. 

A positive sol of hydrous bismuth trioxide results on dialyzing 
a dilute solution of bismuth nitrate containing nitric acid.? 
The sol is but slightly opalescent, is almost neutral, and gives 
only the faintest test for nitrate. A very stable sol is formed by 
adding alkali to a solution of bismuth nitrate in glycerin contain- 
ing Paal’s sodium salts of lysalbinic and protalbinic acids. 
After purification by dialysis, this sol can be evaporated in 
vacuum: at 60°, giving a gel that can be repeptized in water. 

The hydrous gel precipitated from antimony nitrate solution 
with concentrated alkali can be peptized by thorough washing.‘ 
The resulting sol is more stable in the presence of sucrose, man- 
nite, glycerin, and lactose.> The last two substances appear to 
react chemically with the hydrous oxide. 

While the hydrous oxide of bismuth is white, the anhydrous 
oxide is yellow when cold and red when hot. It is used for stain- 
ing glass and porcelain and for neutralizing undesirable colors 
in certain fluxes. 

Higher Oxides of Bismuth.—If a current of chlorine is passed 
into an alkali in which hydrous bismuth trioxide is suspended, a 
reddish powder results which is supposed to be anhydrous or 
hydrous Bi.O.. Similarly, highly oxidized products are formed 
by the electrolytic oxidation of the trioxide and by the action of 
persulfates, hydrogen peroxide, and potassium ferricyanide on 
the trioxide in the presence.of alkali. According to Gutbier and 
Biinz,’ none of these reactions gives a definite homogeneous 
product. 


1 CoRFIELD and Woopwarp: Pharm. J., 113, 83, 128 (1924). 

2 Bittz: Ber., 35, 4434 (1902). 

3 Paau: Pharm. Ztg., 48, 594 (1903). 

4 Ktun and Pirscu: Kolloid-Z. (Zsigmondy Festschrift), 36, 310 (1925). 

5 Cf. Spen and Duar: Kolloid-Z., 33, 193 (1923). 

§ DEICHLER: Z. anorg. Chem., 20, 81 (1899); Hausmr and Vanino: Jbid., 
39, 381 (1904); Rurr: [bid., 57, 220 (1908); Mossr: Ibid., 50, 33 (1906); 
Murr: J. Chem. Soc., 51, 77 (1887). 

7Z. anorg. Chem., 48, 162, 294; 49, 482; 50, 210 (1907); 52, 124 (1907); 
59, 143 (1908), 


THE HYDROUS OXIDES OF THE FIFTH GROUP 279 


Worsely and Robertson! claim to have obtained the tetroxide 
pure, by oxidizing the trioxide suspended in dilute alkali and 
freeing the resulting product from trioxide and alkali by tritu- 
rating with glacial acetic acid. ‘Two isomeric monohydrates are 
described, one brown and the other purplish black. Using con- 
centrated alkali and chlorine, a mixture of yellow tetroxide 
dihydrate and red or brown pentoxide monohydrate is said to 
form. Ammonium persulfate is said to give some hexoxide. 
These observations are incomplete if not inaccurate in many 
respects and should be repeated. It seems altogether unlikely 
that the alleged hydrates are anything but indefinite hydrous 
oxides. 

Unlike the amphoteric oxides of arsenic and antimony, the 
oxides of bismuth are not acid forming in character. 


tJ. Chem. Soc., 117, 63 (1920). 


CHAPTER XII 


THE HYDROUS OXIDES OF MOLYBDENUM, TUNGSTEN, 
AND URANIUM 


Tur Hyprovus OxipEs oF MOLYBDENUM 


Molybdenum Trioxide.—Molybdenum trioxide forms two and 
only two! crystalline hydrates, MoO3;-2H.O and MoO;-H.0. 
The dihydrate separates at room temperature in yellow crusts 
from a nitric acid solution of ammonium molybdate such as is 
used in the estimation of phosphorus. By heating to 40 to 50° a 
solution of the dihydrate or the solid suspended in water, Rosen- 
heim and Davidson? obtained what they called a MoO;3-2H:2O 
to distinguish it from 8 MoO3:H2O which comes down at 65 to 
70°. Both preparations crystallize in fine white needles, but 
the so-called a oxide differs from the 8 in forming with water a 
stable milky suspension or sol and in losing all its hydrate water 
at a lower temperature. It is probable that the differences 
between the two preparations are due to variations in the size 
and physical character of the primary particles thrown down at 
different temperatures, rather than to allotropy. Doubtless 
this could be settled by an x-ray study of the crystal structure of 
the two preparations such as Burger? used to establish the chemical 
individuality of MoO3:H20 and MoOs3. , 

Graham‘ first recognized the existence of a sol of the trioxide 
which he prepared by dialysis of a 5 per cent solution of sodium 
molybdate acidified with a slight excess of HCl. During dialysis 
this sol behaves like hydrous ceric oxide in settling to a jelly 
which subsequently liquefies as the dialysis is continued. The 
sol is very stable toward electrolytes, has a yellow color and an 
astringent taste, and is acid to litmus. By evaporating in vac- 

1 HUTTie and Kourre: Z. anorg. Chem., 126, 167 (1928). 

2Z. anorg. Chem., 37, 314 (1903). 

3Z. anorg. Chem., 121, 240 (1922). 

4 Tiebig’s Ann. Chem., 186, 65 (1865). 

280 


~ MOLYBDENUM, TUNGSTEN, AND URANIUM 281 


uum over sulfuric acid, a glassy hydrous mass is obtained which is 
readily taken up again by water.! Graham’s observations were 
confirmed by Sabanejeff,? Linebarger,? and Lottermoser;? but 
Bruni and Pappada? failed to get a sol by dialysis of a nitric 
acid solution of ammonium molybdate having the composition 
of the phosphoric acid reagent. Rosenheim and Bertheim® 
likewise claimed that the solutions formed by shaking MoO; -- 
2H.O with water are not colloidal since at every temperature, 
the oxide possesses a definite solubility. However, such solu- 
tions saturated at high temperature do not crystallize out on cool- 
ing even when stirred for a long time with crystals of dihydrate. 
Indeed, a solution saturated at 100° is fortyfold supersaturated 
on cooling to room temperature. Cryoscopic determinations 
on such a solution indicate a molecular weight for the 
trioxide in solution of approximately 600, which is of the same 
order as Sabanejeff obtained for the Graham sol. This fact, 
together with the observed high conductivity and high hydrogen 
ion concentration of the dihydrate solution, led to the conclusion 
that solutions of MoO;-2H.0, whether prepared directly or by 
the method of Graham, are not colloidal. This conclusion was 
called in question by Wohler and Engels,’ who demonstrated the 
heterogeneity not only of Graham’s sol but of the nitric acid 
solution of ammonium molybdate and the aqueous solutions of 
MoO;-2H,0 saturated in the hot or in the cold. All of these 
solutions contained particles clearly visible in the ultramicro- 
scope, which were precipitated by the addition of gelatin but 
not by electrolytes. In the light of their observations, Wohler 
and Engels classify the solutions as semicolloidal, since they 
appear to lie in the borderland between true crystalloidal solu- 
tions and hydrophile sols. This disposition of the matter fails 
to emphasize the important fact that MoO;-2H:,0 is soluble to 
a certain extent and, hence, under different conditions, it may 


1Cf. Utuick: Sitzb. Akad. Wiss. Wien, 60, 302 (1870). 
2 Ber., 23, 87 (1890). 

3 Am. of. Sct., (3) 48, 222 (1892). 

4 “Uber anorg. Kolloide,”’ Stuttgart, 11 (1901). 

5 Gazz. chim. ital., 31, I, 244 (1901). 

6 Z. anorg. Chem., 34, 427 (1903); 37, 314 (1904). 

7 Kolloidchem. Bethefte, 1, 466 (1910). 


282 THE HYDROUS OXIDES 


be chiefly in solution or chiefly colloidal. A newly formed 
solution of sodium molybdate acidified with hydrochloric acid 
will contain more oxide in solution than an old preparation, 
since ageing brings about an increase in size and a decrease in 
solubility of the colloidal particles. In all preparations the 
particles are relatively small and the solution pressure sufficiently 
large to make the mixture distinctly acid and a good conductor. 
But a part of the oxide is suspended in the liquid, and this con- 
tributes neither to the acidity nor the conductivity of the solu- 
tion. The lowering of the vapor pressure of such mixtures is due 
in large measure to the dissolved oxide and not to the suspended 
particles; hence, molecular weights deduced from cryoscopic 
measurements under the assumption that all the oxide is dis- 
solved are necessarily erroneous. 

Rosenheim heated to 50° a solution containing hydrochloric 
acid and sodium molybdate in the ratio of 4:1. From this 
solution there precipitated gradually, a slimy hydrous mass 
which was readily peptized by water after thorough washing. 
The particles in this sol are larger and less hydrous than in the 
Graham sol; and, unlike the latter, it is shightly opalescent and 
is precipitated in flocs by the addition of solutions of neutral 
salts and weak acids. 

Dimolybdenum Pentoxide.—The hydrous oxide of pentavalent 
molybdenum is thrown down by adding ammonia to a hydro- 
chloric acid solution of MoO; previously reduced with metallic 
molybdenum! or with hydriodic acid.2, At one time these were 
thought to be oxides of tetravalent molydenum, but Klason® 
obtained a compound with identical properties by adding ammo- 
nia to a dilute solution of (NH,4)eMoOCl; in which the molyb- 
denum is pentavalent. The precipitate is distinctly gelatinous; 
but it is said to have the composition MoO(OH); when dried 
over phosphorus pentoxide. When newly formed, it is very 
similar in physical character and color to hydrous ferric oxide. — 
Like the latter, it is peptized by thorough washing, forming a 
clear sol which varies in color from yellow to dark red depending 


1 BeRzELIUS: Pogg. Ann., 6, 366, 389 (1826). 

2 P&CHARD: Compt. rend., 118, 804 (1894); Ann. chim. phys., (6) 28, 537 
(1893). | 

3 Ber., 34, 148 (1901). 


be 


MOLYBDENUM, TUNGSTEN, AND URANIUM 283 


on the concentration. The sol gives a gelatinous precipitate 
with electrolytes, but it is probable that it could be precipitated 
as a jelly since Berzelius obtained a transparent jelly simply by 
allowing a dark red sol to stand for a month in a closed vessel. 

Freundlich and Leonhardt! studied the properties of a sol pre- 
pared by peptizing the hydrous oxide thrown down with ammonia 
from a very dilute solution containing pentavalent molybdenum. 
The precipitate as well as the sol oxidizes to molybdenum blue 
more readily than Berzelius’ preparation, probably on account 
of the difference in size of the particles. The negatively charged 
sol agglomerates fractionally on adding electrolytes, and the last 
traces of sol are thrown down only by relatively high concentra- 
tions of multivalent cations that ordinarily precipitate in low 
concentrations. This behavior is accounted for by the wide 
variation in the size of the colloidal particles; the larger particles 
agglomerate first, leaving a very dilute, highly dispersed sol which 
is not readily precipitated. In this respect, it is like Péan de St. 
Gilles’ ferric oxide sol.? Like the latter also, the precipitation 
is readily reversible when the precipitating ions are weakly ad- 
sorbed, and almost irreversible when the DATA ate ions are 
strongly adsorbed. 

Molybdenum Dioxide and sangtiocide: —Al]though the oxide 
MoO: can be prepared by oxidizing molybdenum or by reducing 
MoOs3 under suitable conditions, it is claimed that the hydrous 
oxide or hydroxide of tetravalent molybdenum does not exist, 
the reported preparations being hydrous pentoxide.* Paal and 
his coworkers,* however, claim to have prepared the compound 
by reduction, at room temperature, of an ammonium molybdate 
solution with hydrogen in the presence of a little colloidal pal- 
ladium. By stopping the reduction when the theoretical amount 
of hydrogen is used up, there is obtained a greenish-black mass 
which has adsorbed most of the colloidal palladium. If dried 
under suitable conditions, the composition can be made to 
approximate MoO(OH),.; but there is no evidence that this is a 


1 Kolloidchem. Bethefte, 7, 172 (1915). 

2 Weiser: J. Phys. Chem., 25, 672 (1921); 24, 312 (1920). 

3 Kiason: Ber., 34, 153 (1901); GuicHarp: Compt. rend., 143, 744 (1906). 

4Paau and Briniss: Ber., 47, 2214 (1914); Paat and BiTrner: 48, 220 
(1915). 


284 THE HYDROUS OXIDES 


definite hydrate. or that all the molybdenum is tetravalent. If 
the reduction is carried out in the presence of Paal’s sodium pro- 
talbinate, there results a stable sol, black in reflected light and 
reddish brown in transmitted light. By a similar procedure 
Paal claimed to get the precipitated hydrous oxide and sol of 
trivalent molybdenum. The precipitate is an amorphous black 
mass! which cannot be dehydrated to Mo.03 without oxidation 
taking place.’ 

Molybdenum Blue.—When a solution of Mo0; or an acidified 
molybdate is reduced by hydrogen sulfide, sulfur dioxide, stan- 
nous chloride, metallic molybdenum, zinc, or other reducing agent, 
a deep-blue solution results, which deposits a blue hydrous 
precipitate known as molybdenum blue. A similar product is 
obtained by the oxidation of lower oxides such as MoOs, and by 
adding a cold dilute solution of MoO, in hydrochloric acid to a 
solution of ammonium molybdate. The last method of forma- 
tion suggests that the blue compound is a molybdenum molyb- 
date such as MoO2:2Mo0O;; but much doubt exists as to its 
composition. Marchetti® believes it to be Mo30.5-5H2O; Gui- 
chard,* Mo;0Oi4-6H20; Junius,> Mo7O2; while Bailhache® and 
Klason’ believe theré are a number of blues which Klason regards 
as complex derivations of Mo.O0; and MoO; analogous to phos- 
phomolybdic acid. The evidence for the existence of different 
compounds is based largely on analytical differences that are 
probably of the same order of magnitude as the experimental 
errors inherent in analyzing a colloidal mass; hence, whether there 
is an individual blue or a number of related compounds, the 
composition is known only approximately. Marchetti® claims 


1 MuTHMANN and Naas: Ber., 31, 2009 (1898); Cuitesorti: Atti accad. 
Lincei, (5) 12, II 22, 67 (1903); SmirH and Hosxinson: Am. Chem. J., 
7, 90 (1885). 

2 GUICHARD: Ann. chim. phys., (7) 23, 498 (1901). 

3Z. anorg. Chem., 19, 390 (1899) ; MurumMann: Liebig’s Ann. Chem., 
238, 108 (1887). 

4 Ann. chim. phys., (7) 28, 498 (1901); Burzmiius: Pogg. Ann., 6, 385 
(1826). 

5>Z. anorg. Chem., 46, 428 (1905). 

6 Compt. rend., 183, 1210 (1901). 

7 Ber., 34, 153 (1901). 

8 Z. anorg. Chem., 19, 391 (1899). 


—— 


MOLYBDENUM, TUNGSTEN, AND URANIUM 285 


to have prepared a crystalline oxide having the formula Mo30s- - 
5H.O by the cathodic reduction of a hydrochloric acid solution 
of MoO; and subsequent evaporation to crystallization. Cryo- 
scopic investigations on a solution of this oxide in water indicate 
a molecular weight of 460 as compared to 416 for Mo3Os. Bultz! 
was unable to confirm these observations, and Koppel? questions 
the existence of a crystalline molybdenum blue. 

Biltz prepared a stable sol by reduction with hydrogen sulfide 
of an ammonium molybdate solution acidified with sulfuric 
acid, and subsequent dialysis. Dtumanski made _ ultramicro- 
scopic observations on a fairly pure sol prepared in this way and 
observed submicrons in rapid motion, if the sol was not too 
dilute; but on high dilution, it appeared optically empty. 
Dumanski also prepared an electrolyte-free solution by reducing 
pure MoQO3;, suspended in water, with metallic molybdenum. 
This appeared optically empty in the ultramicroscope, and from 
the freezing-point lowering, the molecular weight was calculated 
to be 440(Mo303 = 416). The addition of small amounts of 
electrolytes caused submicrons to appear. Dumanski concludes, 
therefore, that very pure molybdenum blue dissolves in water 
forming a true solution, but the presence of a small amount of 
electrolyte polymerizes the molecules. Since the blue as usually 
prepared contains adsorbed impurities, its solutions are colloidal. 
These observations of Dumanski should be confirmed, for if the 
facts are as stated, they raise the question of the mechanism of 
the agglomeration of monomolecular molecules by the presence 
of small amounts of a variety of electrolytes. 

The sol of molybdenum blue, prepared by Biltz’s method, is 
negatively charged and so is precipitated by positively charged 
hydrous oxide sols of iron, aluminum, chromium, thorium, 
zirconium, and cerium. Particularly beautiful and stable color 
lakes with the hydrous oxides of aluminum, thorium, and cerium 
are formed by the mutual precipitation of the oppositely 
charged sols. 

Sols of molybdenum blue act as a dye bath, imparting a blue 
color to various fibers. Builtz* studied the influence of concen- 

1 Ber., 37, 1095 (1904). 


: bette ‘‘Handbuch anorg. Chem.,”’ 4, (2) 626 (1921). 
3 Ber., 37, 1766 (1904); 38, 2963 (1905). 


286 THE HYDROUS OXIDES 


tration of sol on the amount of blue oxide taken up by silk, 
cotton, and hydrous aluminum oxide. The results of these 
observations are given in Fig. 18. The uniform nature of the 
curves shows that the colored fibers and lake are ‘‘adsorption 
- compounds,”’ the composition of which varies continuously 
with the concentration of sol. The isotherms of adsorption are 
very similar to those obtained for organic colloidal dyestuffs 
such as benzidine! and benzopurpurine? and for dyes in true 





0 0 
0 0.25 0.50 0.75 1.00 1.25 150 tS eee 
Concentration of Molybdenum Blue, per cent 


Grams Molybden um Blue Adsorbed by One Gram of Alumina 
Grams Molybdenum Blue Adsorbed by One Gram of Fibers 


Fic. 18.—Adsorption of molybdenum blue by silk, cotton and hydrous alumina. 


solution such as picric acid’ and Congo red. In view of these 
observations, there is no longer any question but that dyeing by 
many organic and inorganic dyestuffs is essentially an adsorption 
process rather than a chemical process of the usual type, or a 
solid-solution phenomenon. 

With true solutions of dyes, one is not surprised to find a 
continuous increase in the amount of dye taken up with increasing 
concentration of the dye bath, in accord with the adsorption 
isotherm. But in colloidal solutions of dyes, the particles con- 


1 Groraievics: Monatshefte fiir Chemie, 15, 705 (1894); 16, 345 (1895). 
2 FREUNDLICH and LosEv: Z. physik. Chem., 59, 284 (1907). 
3 APPLEYARD and WauLkER: J. Chem. Soc., 69, 13834 (1896). 


MOLYBDENUM, TUNGSTEN, AND URANIUM 287 


stitute a second phase in the ordinary sense of the term; and it is 
not obvious why they should fail to act like a phase of constant 
composition instead of the adsorption varying continuously with 
the apparent concentration of the bath. Bancroft' has gotten 
- around this difficulty by accepting with reservations, the physicist 
view that, according to the kinetic molecular theory, a suspended 
particle should behave like a molecule in true solution. If this is 
true, it will account for the marked similarity in the isotherms of 
adsorption for colloidal and molecular solutions. 

While recognizing the value, at some times, of treating a 
colloidal solution as having some of the properties of a true solu- 
tion, Bancroft emphasizes the importance of distinguishing 
between the two at other times. In the same way, it is useful 
to treat a solid solute as behaving in certain respects like an ideal 
gas, but this does not mean either that the solute behaves in all 
respects like an ideal gas or that it is an ideal gas. In most 
cases the distinction between true and colloidal solutions can be 
made by applying the criterion of Gibbs. According to Gibbs, 
an apparent phase is not a one-phase system unless the properties 
are definitely defined when the temperature, pressure, and con- 
centration are fixed. By applying this test, it may be shown 
readily that most colloidal solutions are two-phase systems. 
The difficulty comes with solutions of such substances as tannin 
and soaps which appear to satisfy Gibbs’ criterion for a one-phase 
system when in reality they are two-phase systems. A similar 
situation is encountered with a mixture of two gases which in 
the last analysis is neither physically nor chemically homogeneous 
but which is a one-phase system, nevertheless. To take care of 
these cases, Bancroft assumes that any gas or vapor will pass 
through any pore through which any other gas or vapor will 
pass. If this unproved but reasonable assumption is granted, 
it leads directly to the conclusion that any substance which can 
be filtered out by an ultrafilter is in colloidal solution and not 
in true solution, an ultrafilter being defined as a porous membrane 
which shows no marked negative adsorption, that is, specific 
adsorption of the solvent. This criterion puts soap and tannin 
in the list of colloidal solutions and would undoubtedly take care 
of the highly dispersed solutions of molybdenum trioxide and 


1 J. Phys. Chem., 29, 966 (1925). 


288 THE HYDROUS OXIDES 


molybdenum blue. It should be pointed out, however, that any 
apparent solution which will pass through the finest ultrafilter 
is not necessarily in true solution. 


Tur Hyprovus OxIpEs or TUNGSTEN 


Anhydrous tungsten trioxide does not combine with water; 
but an insoluble monohydrate usually known as tungstic acid, 
H.WO,, is formed by precipitating a solution of a tungstate with 
excess mineral acid at the boiling point. The hydrate comes 


ENS 





























Mols Water to One Mol WO, 


oO 
om 








0 —— 
0 | 62.5 125.0 187.5 250.0 312.5 
Temperature 





Fig. 19.—Dehydration curves of yellow and white tungsten trioxide. 


down as a yellow powder possessing a definite crystalline structure 
distinctly different from that of WO;.! By carrying out the 
reaction at low temperatures, the oxide comes down as a white 
voluminous mass that cannot be washed completely free from 
adsorbed salts. The white oxide was believed to be WQO3-- 
2H.O; but this seems improbable in the light of Hiittig and 
Kurre’s” recent investigations of the change in vapor pressure of 
different preparations with changing temperature, using a spe- 

1 BurGcerR: Z. anorg. Chem., 121, 240 (1922). 

2Z. anorg. Chem., 122, 44 (1922). 


MOLYBDENUM, TUNGSTEN, AND URANIUM 289 


cially designed tensi-eudiometer.! The results of observations on 
both yellow and white preparations are given in the temperature- 
composition curves reproduced in Fig. 19. The yellow oxide 
forms one and only one hydrate WO;3:-H.2O, and the white 
voluminous compound is a hydrous oxide, the water content 
varying continuously with change in temperature. The curve 
for the yellow oxide is reproducible; but the ease with which the 
white compound gives up its water is determined by the size of 
the hydrous particles. Thus, the curve for the very highly dis- 
persed 6 oxide lies above that for the coarser a oxide throughout 
the entire range of the investigation. The crystal structure of 
the white hydrous oxide is apparently different from that of the 
yellow monohydrate or yellow WO3. If this be true, it means 
that the oxide exists in two forms and not that the white com- 
pound is a hydrate, as Burger? supposed. The white oxide 
becomes yellow on standing;? and the yellow oxide sometimes 
takes on a greenish color which has been traced to the presence 
of a lower oxide, possibly W203. 

Graham’? first prepared a sol of tungstic acid, so called, by dialysis 
of a 5 per cent solution of sodium tungstate acidified with only 
a slight excess of dilute hydrochloric acid. On evaporating the 
sol to dryness, a glassy mass was obtained which could be heated 
to 200° without losing its sol-forming property. An 80 per cent 
sol with a density of 3.25 was obtained by peptizing the glassy 
mass with one-fourth its weight of water. The sol is less stable 
than the corresponding molybdenum trioxide sol and precipi- 
tates out in the form of beautiful quadratic prisms,® after 
standing several months. 

The Graham sol cannot be prepared free from alkali, which 
led Sabanejeff’ to conclude that it is a solution of a sodium salt 
of the formula Na,.O:4WO;3. This seems not to be the case, 
since Biltz and Vegesack® found the ratio Na2O : WO; to be 2 : 11 

1 Hirrie: [bid., 114, 161 (1920). 

2Z. anorg. Chem., 121, 240 (1922). 

3 Moser and Euruicu: Kdel-Erden u.-Erze, 3, 49, 65 (1922). 

4Van Liempt: Z. anorg. Chem., 119, 310 (1921). 

5 Pogg. Ann., 123, 539 (1864). 

6. WoOuLER and Enaets: Kolloidchem. Bethefte, 1, 472 (1910). 

7Z. anorg. Chem., 14, 354 (1897). 

8 Z. phys. Chem., 68, 376 (1910), 


290 THE HYDROUS OXIDES 


in a well-dialyzed sol. Wohler and Engels! confirmed Graham’s 
observations and demonstrated the optical heterogeneity of the 
preparation. Unlike colloidal MoQOs, the sol is not precipitated 
by adding gelatin, but an adsorption complex is formed which is 
thrown down by the addition of a little ammonium chloride. 
The presence of a small amount of tungsten trioxide in colloidal 
molybdenum trioxide seems to act as a protective colloid,? 
preventing the precipitation of the latter by gelatin. Similarly, 
molybdenum trioxide seems to exert a protecting action on tung- 
sten trioxide. As has been pointed out, molybdenum trioxide 
comes down only very slowly from a strongly acidified molybdate 
solution in the cold, whereas tungsten trioxide precipitates readily 
from a strongly acidified tungstate. If a mixture containing a 
small amount of tungstate and a large amount of molybdate is 
acidified, no precipitate forms for a long time unless the mixture 
is warmed, the time required for its appearance at a given tem- 
perature being determined by the composition of the mixture. 
The precipitate is an adsorption complex since the two oxides 
exhibit a mutual adsorption for each other. ‘The more stable 
molybdenum trioxide sol adsorbs and so holds tungsten trioxide 
in colloidal solution until the particles of the latter grow to the 
point of precipitation, carrying down with them adsorbed molyb- 
denum trioxide. 

Although the nature of Graham’s dialyzed solutions has been 
questioned, there can be no doubt as to the colloidal character 
of the solution of the yellow oxide formed by mechanical dis- 
integration® or of the white hydrous oxide peptized by washing. 
The white oxide always comes down as a gelatinous precipitate 
when sodium tungstate is treated in the cold with excess acid. 
The velocity of precipitation and the nature of the precipitate 
depend on the hydrogen ion concentration of the acid used. 
Contrary to von Weimarn’s theory, Lottermoser® found the precip- 


1 Kolloidchem. Bethefte, 1, 472 (1910); cf. PappapA: Gazz. chim. ital., 32, 
(2), 22 (1902). 

2 Cf. KréaeEr: Kolloid-Z., 30, 18 (1922). 

3 WEGELIN: Kolloid-Z., 14, 65 (1914). 

4LOTTERMOSER: Kolloid-Z., 15, 145 (1914); cf. vAN LizmptT: Rec. trav. 
chim., 43, 30 (1924). 

5 Van Bemmelen’s: ‘‘Gedenkboek,”’ 152 (1910). 


MOLYBDENUM, TUNGSTEN, AND URANIUM 291 


itate to be more voluminous the slower the rate of precipitation. 
On washing the gel by decantation, it gradually becomes less 
voluminous and yellow in color, and finally is peptized completely, 
forming a yellow very cloudy sol in which the particles appear 
rod shaped.! Lottermoser compares Graham’s clear stable 
tungsten trioxide sol with the latter’s clear ferric oxide sol formed 
in a similar way, and the cloudy tungsten trioxide sol with Péan 
de St. Gilles’ ferric oxide sol. There is some doubt as to whether 
the comparison is justified. The difference between the two 
ferric oxide sols is due to a difference in the size and hydrous 
character of the particles, whereas the tungsten oxide sols may 
be different chemically. The x-ray investigations which indicate 
the chemical individuality of white and yellow tungsten trioxide 
should be confirmed. 

Pappada? prepared a very sensitive sol by peptizing the tri- 
oxide with oxalic acid and purifying by dialysis; and Miiller? 
obtained a sol highly sensitive to electrolytes by diluting with 
water an alcohol-ether solution of the oxide. 

Tungsten Blue.—The first product formed on reducing tung- 
sten trioxide or a tungstate is a blue substance similar to molyb- 
denum blue and known as tungsten blue. It may be formed 
also by partial oxidation of tungsten dioxide or of the hydrolysis 
product of tungsten pentachloride and pentabromide. The 
composition of the blue has been represented by a number of 
formulas ranging from W.20;* to W;O.4;> but the bulk of the 
evidence indicates that it is a mixture of variable composition 
and not a single chemical individual. Depending on the 
method of preparation, the color varies from purple bronze to 
deep blue; but it is not known whether this is due to differences 
in composition or physical structure or both. 

A sol of tungsten blue is formed by neutralization with ammo- 
nia of a solution of metatungstic acid saturated with hydrogen 

1 DimsseLHorst and Freunpuicu: Phystk. Z., 17, 117 (1916). 

2 Gazz. chim. ital., 32, II 22 (1902). 

3 Van Bemmelen’s: ‘‘Gedenkboek,”’ 416 (1910). 

4 CHAUDRION: Compt. rend., 170, 1056 (1920); Ann. chim., 16, 221 (1921); 
VAN Liempt: Z. anorg. Chem., 126, 183 (1923). 

5’ ALLEN and GorrscHaLK: Am. Chem. J., 27, 328 (1902). 

6 WOHLER and Batz: Z. Elektrochem., 27, 406 (1921); RernpERS and 
VeRvVLOET: Rec. trav. chim., 42, 625 (1928). 


292 THE HYDROUS OXIDES 


sulfide,! or by electrolytic reduction of an acidified tungstate 
solution.2 In the purification by dialysis, appreciable amounts 
of the blue substance pass through the dialyzing membrane; 
the sol remaining has a deep sky-blue color in marked contrast 
to the slate-blue color of the corresponding molybdenum sol. 
Like the latter, however, it is negatively charged and is precipi- 
tated by electrolytes and positively charged sols. Freshly 
prepared sols dye silk, cotton, and wool directly, imparting a 
clear-blue color to the fiber.® 


THE Hyprovus OxipEs oF URANIUM 


Uranium Trioxide.—Graham/ first prepared a sol of uranium 
trioxide by adding potassium carbonate to a uranyl salt solution 
containing sugar, and dialyzing. ‘The deep orange-yellow col- 
loid was very stable but was readily agglomerated by electro- 
lytes. It is not clear why Graham found it necessary to use 
sugar in the preparation of his sol, for Szilard® found that uranyl 
nitrate peptizes the oxide directly. To get the oxide inasuitable 
form, Szilard mixed a solution of uranyl acetate with ether and 
exposed the mixture to light, thereby obtaining a hydrous violet 
precipitate® analogous to molybdenum blue but having a definite 
composition U;Os. On allowing the thoroughly washed oxide 
to stand for a day, it oxidized to the yellow trioxide which was 
suspended in water and added to a hot solution of uranyl nitrate 
as long as it was peptized easily. The orange-yellow sol was 
quite stable in the presence of an appreciable excess of uranyl 
salt; but if too much oxide was added, it agglomerated spontane- 
ously in a form that was not readily repeptized. 

It is altogether probable that the sol is a hydrate rather than 
a hydrous oxide since the anhydrous oxide takes up water at 


1 ScHEIBLER: J. prakt. Chem., 80, 204 (1860); 88, 273 (1861); Bizz: Ber., 
37, 1095 (1904). 

> LetserR: Z. Elektrochem., 18, 690 (1907); Kroger: Kolloid-Z., 30, 16 
(1922). 

§ BrutTz: Ber., 37, 1771 (1904). 

4 Phil. Trans., 151 I, 183 (1861). 

5 J. chim. phys., 5, 488, 495, 6386 (1907). 

® Atoy: Bull. soc. chim., (2) 25, 344 (1901); ALoy and Roprmr: Jbid., (4) 
27, 101 (1920). 


MOLYBDENUM, TUNGSTEN, AND URANIUM 293 


room temperature,! forming a dihydrate that is converted into 
a monohydrate at a temperature of 80° and a water-vapor pres- 
sure of 15 millimeters.?, The hydrate is usually yellow in color, 
but an orange compound of the same composition is obtained 
by electrolysis of uranyl nitrate? or by suspending the violet 
hydrous U3;Ogs in water which is subsequently boiled in the air. 
The anhydrous oxide formed by decomposition of uranyl nitrate 
is yellow if the decomposition takes place slowly, but red if the 
decomposition is rapid.> It is probable that the variations in 
color of both the anhydrous and hydrated oxides are the result 
of differences in physical character of the same compound formed 
in different ways. 

Uranium Dioxide.—Hydrous uranium dioxide is thrown down 
as a voluminous red-brown mass by adding alkalies or ammonia 
to a cold green uranous solution. The gel loses water and 
becomes denser and darker on heating. It is probable that the 
newly formed hydrous mass would be peptized by washing, since 
Samsonow’ obtained a sol by washing the dark hydrous dioxide 
precipitated during the electrolytic reduction of 50 grams of 
uranyl chloride in 100 cubic centimeters of 2 N hydrochloric acid. 
Samsonow’s sol when freshly prepared contains small positively 
charged particles in brisk Brownian movement. The particles 
grow quite rapidly, however, and within 24 hours most of the 
oxide settles out. 

It is of particular interest that colloidal uranium trioxide 
appears to catalyze the formation of formaldehyde by the action 
of sunlight on a solution of carbon dioxide in water.® 


1LEBEAu: Compt. rend., 154, 1808 (1912). 

2 Hirria and ScHROEDER: Z. anorg. Chem., 121, 243 (1922). 

3 OECHSNER DE ConinckK: Bull. Acad. Roy. Belg., 222 (1901). 

4 OECHSNER DE CoNINCK: Compt. rend., 132, 204 (1901); Bull. Acad. Roy. 
Belg., 363, 448 (1904). 

5 Anoy: Bull. soc. chim., (3) 28, 368 (1900). 

6 Anoy: Bull. soc. chim., (3) 21, 613 (1899). 

7 Kolloid-Z., 8, 96 (1911). 

8 Moore and Wesster: Proc. Roy. Soc., 87B, 163 (19138); 90B, 168 
(1919), 


CHAP TER XT 
THE HYDROUS OXIDES OF MANGANESE 


Hyprous MANGANESE DIOXIDE 


Hydrous manganese dioxide is obtained by the oxidation of a 
manganous salt with such oxidizing agents as permanganate, 
hypochlorite, chlorate, ammonium persulfate, nitric acid, and 
ozone. It is also obtained by hydrolysis of a salt of tetravalent 
manganese and by reduction of permanganate by hydrogen 
peroxide, glycerin, dextrose, potassium oxalate, etc. It is 
difficult, if not impossible, to obtain the hydrous oxide in a pure 
form, partly on account of its tendency to lose a portion of its 
oxygen giving mixtures of MnO and MnO,! and partly because 
of its high adsorption capacity. 

While definite hydrates of precipitated manganese dioxide 
have been described, the composition was found by van Bemme- 
len? to be indefinite, depending on the physical character, age, 
and conditions of drying the sample. Two widely different 
preparations were studied by van Bemmelen, one the ordinary 
black compound precipitated from a solution of manganous 
salt by alkali hypochlorite, and a second red variety obtained by 
hydrolysis of Mn(SO,)o.2 The red oxide is much more finely 
divided than the black and possesses a much higher adsorption 
capacity for water and dissolved electrolytes. Both oxides 
show a strong tendency to adsorb hydroxyl ion. This is evi- 
denced by the fact that dilute solutions of alkali peptize the gel 
forming a stable negative sol. Moreover, neutral salts such as 
potassium sulfate, chloride, and nitrate are hydrolyzed by the gel | 


1Wricut and Menke: J. Chem. Soc., 37, 25 (1880); Goocu and Austin: 
Am. J. Sct., (4) 5, 260 (1898); Goraru: Compt. rend., 110, 1134 (1890); 
von Knorre: Z. angew. Chem., 14, 1149 (1901). 
2 J. prakt. chem., (2) 28, 324, 379 (1881). 
3 Fremy: Compt, rend., 82, 475, 1231 (1876), 
294 


THE HYDROUS OXIDES OF MANGANESE 295 


owing to stronger adsorption of base than of acid, the hydrolytic 
decomposition being more complete when the salt solutions are 
dilute. Thus, 65 per cent of the potassium present in a 0.0025 
N solution of potassium sulfate is adsorbed by the hydrous oxide 
but only 6 per cent of that from a 0.1 N solution. The total 
amount adsorbed, however, increases with increasing concen- 
tration of salt.1. This behavior accounts for Gorgeu’s? observa- 
tion that solutions of both alkali and alkaline earth salts become 
acid when brought in contact with the hydrous oxide. In view 
of this marked adsorption capacity for alkalies, it is probable 
that the so-called manganites* formed by precipitating hydrous 
manganese dioxide in the presence of basic oxides, are not definite 
compounds. In favor of this conclusion, it may be pointed out 
that the composition is determined by such variable factors as 
the physical character of the hydrous mass, the concentration of 
alkah, and the method of washing the precipitate. 

Manganese Dioxide Sols.—As previously mentioned, van 
Bemmelen‘ observed the ease with which hydrous manganese 
dioxide is peptized by dilute alkali. He also noted that the 
precipitated oxide is more or less completely peptized by washing 
out the excess of adsorbed alkali or salt. The voluminous red 
oxide is very readily peptized by washing, forming a clear-brown 
sol that is quite sensitive to the action of electrolytes. 

The most satisfactory method of preparing the sol consists in 
reducing potassium permanganate under such conditions that 
the precipitation concentration of no electrolyte in the solution 
is exceeded. The solution becomes alkaline during the reduction; 
but, as already noted, an appreciable concentration of hydroxyl 
ion is favorable to sol formation. Reducing agents that have 
been employed successfully are hydrogen peroxide, sodium thio- 
sulfate, arsenious acid, reducing sugars, and ammonia. 


1 HUMMELCHEN and Kapprmn: Z. Pflanzenerndhr. Diingung, 3A, 289 
(1924); Chem. Abstracts, 19, 1800 (1925). 

2 Ann. chim. phys., (3) 66, 155 (1862). 

3 Rousseau: Compt. rend., 104, 780, 1796 (1887); 114, 72 (1892); Sor- 
sTEIN: Pharm. Ztg., 32, 659 (1887); Stinet and Marawskt: J. prakt. Chem., 
(2) 18, 86 (1878); GuAsEeR: Monatshefte fiir Chemie, 6, 329 (1885); 7, 651 
(1886). 

4Cf. also GorarEu: Ann. chim. phys., (3) 66, 154 (1862); Serine: Ber., 
16, 1142 (1883); Sprinc and Dr Borck: Bull. soc. chim., (2) 48, 170 (1867). 


296 THE HYDROUS OXIDES 


Swiontkowski! first reported the formation of a coffee-colored 
sol of manganese dioxide by reducing a solution of KMnO, 
with pure neutral hydrogen peroxide. According to Bredig and 
Marck,’ a satisfactory sol results if a dilute hydrogen peroxide 
solution is added slowly with constant shaking to a potassium 
permanganate solution no stronger than M /10 until the color of the 
permanganate just disappears. By dialysis with conductivity 
water, the conductivity of the sol may be reduced to that of 
ordinary distilled water. A dilute sol is clear yellow in color, 
changing to dark brown as the concentration increases. If not 
too concentrated, the sol can be kept indefinitely without 
precipitating; but it is very sensitive to the action of electrolytes 
with the exception of potassium hydroxide and permanganate. 

Bredig and Marck made a quantitative study of the catalytic 
decomposition of hydrogen peroxide by colloidal manganese 
dioxide. The reaction is of the first order, but in most cases the 
constant increases as the decomposition progresses, possibly 
owing to the formation and subsequent decomposition of a 
hydrogen peroxide salt during the course of the reaction.? In 
alkaline solution, the velocity of decomposition increases to a 
maximum with increasing concentration of hydroxyl and then 
falls off just as Bredig found with colloidal metals. The catalytic 
activity of the oxide is increased by heating the sol for 4% hour, 
but prolonged heating causes precipitation. The presence of 
gelatin increases the stability of the sol and raises its catalytic 
activity slightly. Low concentrations of substances like hydro- 
gen sulfide, potassium cyanide, and carbon dioxide which have a 
marked poisoning action on a platinum catalyst, are without 
effect on manganese dioxide. On the other hand, phosphorus, 
its oxidation products, and sodium phosphate cut down the 
catalytic activity of the oxide, and mercuric chloride and potas- 
sium fluoride increase it. 

A mixture of perborate and permanganate may be added to 
bath water to make what is known as an “oxygen bath.”’ The 
reaction in solution gives hydrogen peroxide and colloidal man- 
ganese dioxide, and the latter catalyzes the decomposition of 

1 Liebig’s Ann. Chem., 141, 205 (1867). 


2 Van Bemmelen’s: ‘‘Gedenkboek,’’ 342 (1910). 
3 Cf., however, LoOTTERMOSER and LEHMANN: Kolloid-Z., 29, 250 (1921). 


THE HYDROUS OXIDES OF MANGANESE 297 


former, setting free oxygen which forms a supersaturated solution 
in the water and is subsequently evolved in small bubbles on the 
skin of the bather. The presence of electrolytes in the bath water 
was found to have such a marked effect on the rate of evolution 
of oxygen that Lottermoser! investigated quantitatively the influ- 
ence of various alkalies and salts on the decomposition of hydro- 
gen peroxide by manganese dioxide. ‘The impurity was added to 
the peroxide solution, after which the catalyst was prepared 
directly in the solution by adding permanganate and base in the 
order named. With salts of a common anion, different cations 
influenced the decomposition in the order Ba’ > Sr” > Ca’’- 
> Na’ > K’ > Li, barium ion accelerating it the most and lith- 
ium ion retarding it the most. Unfortunately, Lottermoser did 
not inquire into the reason for the accelerating action of certain 
electrolytes and the inhibiting action of others; but it is probable 
that this is very closely related to the adsorbability of the cations 
and to the variation in physical character of the hydrous parti- 
cles formed in the presence of different electrolytes. Mg’ and 
NH, ions were found to have a marked retarding action by 
cutting down the concentration of hydroxyl ion. For preparing, 
a satisfactory oxygen bath, it is obviously necessary to avoid 
waters containing magnesium salts. 

The reduction of permanganate by arsenious acid was shown 
by Deisz? to give a very stable sol, particularly if it is not sub- 
jected to dialysis. If evaporated to dryness over sulfuric acid, 
a residue is obtained which is again converted into a sol by shak- 
ing with water. If a bit of sol is allowed to drop into still water, 
beautiful vortex rings are formed. ‘The first ring increases to a 
certain size and breaks into several small rings and these in turn 
into others. All these rings are connected with each other by 
thin lines of hydrous manganese dioxide, thus giving the whole 
system a striking clustered or festooned appearance. If the sol 
is dropped into a salt solution, it is precipitated in the form of 
miniature rings; by using a very dilute solution, the system of 
rings will form before coagulation takes place. This phenomenon, 
first described by Thomson and Newall,’ is not limited to col- 

t Kolloid-Z., 29, 250 (1921). 

2 Kolloid-Z., 6, 69 (1910); cf. Travers: Bull. soc, chim., 37, 456 (1925). 

3 Proc. Roy. Soc., 39, 417 (1886). 


298 THE HYDROUS OXIDES 


loids like milk, ink, blood, soap, etc., but is shown by many 
solutions of both electrolytes and non-electrolytes. Suitable 
concentrations of permanganate give strikingly beautiful rings. 

A stable sol may be obtained by the oxidation of a manganous 
salt in the presence of a protective colloid,! such as albumin, 
dextrin, gum arabic, sodium ‘‘salts” of albuminous products 
and starch. Low concentrations of positive hydrous ferric oxide 
sols precipitate the negative manganese dioxide sol; but in 
high concentrations they adsorb it and hold it in the suspended 
form. Thus, by dissolving a manganous salt in a neutralized 
ferric chloride solution and treating with potassium permanga- 
nate, a dark-brown hydrosol is obtained; similarly, a ferrous 
chloride solution can be oxidized with a potassium permanganate 
solution without any precipitation taking place. 

Witzemann? prepared colloidal manganese dioxide by incom- 
plete oxidation of a glucose solution in the presence of a little 
alkali. On adding slowly 100 cubic centimeters of 6 per cent 
potassium permanganate to a cold solution containing 5 grams 
of glucose in 20 cubic centimeters, together with a few cubic 
centimeters of 10 per cent sodium hydroxide, the mixture rapidly 
became viscous and in 5 to 10 minutes set to a stiff jelly. The 
jelly soon started to synerize, and after standing for a few days, 
it was transformed completely into a stable limpid sol. The rate 
of transformation from the jelly to the sol stage depends on the 
alkali concentration. With quite low concentrations, the jelly 
forms slowly and does not liquefy, while with relatively high 
concentrations, the jelly stage is not observed, the solution 
merely undergoing an initial increase in viscosity, followed by a 
rapid transformation to the limpid sol. This behavior of col- 
loidal manganese dioxide is very similar to that of colloidal 
hydrous ceric oxide*® except that in the latter case the trans- 
formation from a hydrous sol to a jelly and again to a less 
hydrous sol takes place in the absence of glucose. It is obvious 
that the newly formed particles of manganese dioxide are in an 
extremely finely divided and highly hydrous form, and in rela- 


1'TrILLAT: Compt. rend., 188, 274 (1904); Bull. soc. chim., (3) 31, 811 
(1904); German Patent 227491. 

2 J. Am. Chem. Soc., 37, 1079 (1915); 39, 27 (1917). 

3 See p. 255. 


THE HYDROUS OXIDES OF MANGANESE 299 


tively high concentration, they adsorb and entangle all of the 
liquid, forming a jelly. Now, asa rule, inorganic jellies synerize, 
particularly in the presence of salts,! the hydrous particles losing 
water and growing until they settle out. This is particularly 
noticeable with barium sulfate and certain arsenate jellies. But 
in the case of hydrous manganese dioxide, the primary particles 
coalesce to form larger primary particles even though agglomera- 
tion and precipitation are prevented by the protective action of 
glucose and the peptizing action of adsorbed hydroxylion. The 
ageing of the primary particles in the presence of electrolytes 
has, however, left them relatively free from adsorbed water, the 
latter merely serving as the medium in which the slightly hydrous 
particles are suspended. Because of this irreversible change, 
neither an aged ceric oxide or manganese dioxide sol can be 
precipitated in the form of a jelly. Jellies of the hydrous oxides of 
iron and aluminum may be broken up by shaking, forming sols 
of relatively low viscosity which, on standing, set again to firm 
jellies. In these instances the primary particles retain their 
small highly hydrous character in contradistinction to the 
behavior of ceric oxide and manganese dioxide, and the structure 
broken up by shaking gradually re-forms, entangling all of the 
unadsorbed water. 

Probably the simplest method of preparing manganese dioxide 
sols consists in adding concentrated ammonia, one drop every 3 
minutes, to an M/20 solution of potassium permanganate at 90° 
until all the permanganate is reduced.” The only impurity in 
the sol is potassium hydroxide, which has a marked stabilizing 
effect. 

Adsorption by Hydrous Manganese Dioxide.—Dhar and his 
collaborators? have studied the precipitation by electrolytes of 
a manganese dioxide sol in the presence of a protective colloid, 
gelatin. They have also made adsorption studies on the oxide 
precipitated in the presence of various salts. As a result of 


1 Poma and Patront: Z. physik. Chem., 87, 196 (1914). 

2 Guy: J. Phys. Chem., 25, 415 (1921). 

3 CuHatTrerRsi: Proc. Eighth Ind. Sci. Congress, 17, 180 (1921); GaNauULY 
and Duar: J. Phys. Chem., 26, 701, 836 (1922); CHatrerst and Duar: 
Kolloid-Z., 38, 18 (1923); Duar, Sen, and Guosn: J. Phys. Chem., 28, 457 
(1924); cf. Sprrinc and Dr Borckx: Bull. soc. chim., (2) 48, 170 (1887). 


300 THE HYDROUS OXIDES 


these observations, the conclusion is reached that an ion which 
has a high precipitation value for a colloid is most strongly 
adsorbed by the colloid and vice versa. ‘‘'Thus, the monovalent 
ions—silver, sodium, and lithium—are more adsorbed (by 
MnO.) than any of the bivalent, trivalent, or tetravalent ions. 
These facts show that ions of higher valence which in general 
have greater coagulating power are adsorbed the least.”’ Dhar’s 
observations were not made on a purified sol but on hydrous 
manganese dioxide formed by mixing potassium permanganate 
and manganous sulfate in the presence of various electrolytes. 
In the solution from which the oxide separated, there were the 
two reacting electrolytes, the salt whose adsorption was meas- 
ured, together with the soluble products of the reaction, potas- 
slum acid sulfate and sulfuric acid. This makes an almost 
hopelessly complicated system; and it seems unsafe to 
draw any conclusions whatsoever from the observations until 
more is known of the effect of foreign electrolytes on the rate of 
precipitation and physical character of the precipitate, and until 
something is known of the influence of other salts in the system on 
the adsorption of the salt investigated. To cite but one example: 
Aluminum nitrate is adsorbed about eight times as strongly as 
aluminum sulfate, whereas the sulfates of cobalt, copper, and 
cadmium are each adsorbed somewhat more than their respective 
nitrates. Aluminum is not “far less adsorbed” than strontium, 
nickel, cobalt, zinc, barium, or cadmium if the values for the 
nitrates are compared. Pavlov! showed that the taking up 
of silver from nitrate solution by hydrous manganese dioxide is 
not a simple case of adsorption but is complicated by a chemical 
reaction between the adsorbent and the adsorbed salt. With 
colloidal manganese dioxide, as with other sols, it is probable that 
the most readily adsorbed ion will be found to precipitate in 
lowest concentrations, provided influences other than the adsorb- 
ability of the precipitating ions can be suppressed or eliminated.’ 

The adsorption of iron, nickel, and copper by hydrous manga- 
nese dioxide, formed in acid solution by the action of (NH4)2S20, 
on a manganese salt, follows the Freundlich adsorption iso- 

1 Kolloid-Z., 35, 375-877 (1924). 


2G Werte J. Phys. Chem., 29, 955 (1925); cf. ScHiLow: Z. nhyate. 
Chem., 100, 425 (1922). 


THE HYDROUS OXIDES OF MANGANESE 301 


therm, the amounts of the several ions taken up being appreci- 
ably greater at lower concentrations. ! 

Manganese compounds have been found to play an important 
role in many biochemical actions, and in certain instances, this 
may be due to colloidal oxides of manganese.?, Thus manganese 
compounds stimulate alcoholic fermentation® and the growth 
of fungi.4 The stimulating effect on the growth of plants in 
general’ is evidenced by the fact that the production of wheat 
per acre may be increased 10 per cent by sprinkling a manganese 
compound on certain soils. For this purpose, manganese dioxide 
is one of the most effective compounds. Salts of manganese 
likewise appear to stimulate metabolism and to increase the 
hemogenetic power.® It is, therefore, proposed to administer 
manganese therapeutically along with iron to make the latter 
effective. Further, the addition of minute amounts of manga- 
nese increases the activity of the enzyme laccase,’ and colloidal 
manganese dioxide behaves like an oxidase toward guiac tincture, 
hydroquinone, etc.* As the processes mentioned are thought to 
be regulated by enzymes and enzymes are colloidal, Witzemann 
suggests that the effect of manganese on enzymic activity is 
due to the effect of the hydrous oxide on the physical character 
of the enzyme. Thus, if the colloidal oxide keeps the colloidal 
enzyme dispersed under conditions which would normally be 
unfavorable to this effect, then it might be expected to have a 
positive influence on the enzymic activity. 


1GrLoso: Compt. rend., 176, 1884 (1923); 178, 1001 (1924); Bull. soc. 
chim., 37, 641 (1925). 

2 WiTzEMANN: J. Am. Chem. Soc., 37, 1089 (1915). 

3 Kayser and Marcuanp: Compt. rend., 145, 343 (1907). 

4 BERTRAND and JAVILLIER: Bull. soc. chim., 11, 212 (1912); Burrranp: 
Ibid., 11, 494 (1912); Warerman: J. Chem. Soc., 104, I, 229 (1913). 

5 Masoni: Staz. sper. agrar. ital., 44, 85 (1911); Monremartini: [bid., 
44, 564 (1911); Ricct and Barpera: Ibid., 48, 677 (1915); BarTMann: 
J. agr. prat., (2) 20, 666 (1911); Skinner and Suuuivan: U. S. Dept. 
Agr. Bull. 42, (1913); Preirrer and Biancx: Landw. Vers.-Sta., TT, 33 
(1912); 88, 257 (1914). 

6 PrccinIN1: Eighth Int. Cong. Applied Chem., 19, 263 (1912); Biochem. 
terap. sper., 2, 885 (1910-1911); Chem. Abstracts., 7, 369 (1918). 

7 BertTRAND: Bull. soc. chim., (3) 17, 619, 753 (1897); Ann. chim. phys., 
(7) 12, 115 (1897); cf. also Bacu: Ber., 43, 364 (1910). 

8 SyotLEMA: Chem. Weekblad, 6, 287 (1909); Chem, Zentr., I, 496 (1911), 


302 THE HYDROUS OXIDES 


In addition to its use in the manufacture of chlorine and as a 
depolarizer in the Le Clanche battery, manganese dioxide in the 
anhydrous or slightly hydrous state finds wide application as a 
dryer for oils in paints. The drying is a process of oxidation 
and the manganese dioxide serves as an efficient oxygen carrier or 
catalyst. It is sometimes used also in conjunction with other 
oxides to produce warmer shades in colored glass and to destroy 
the injurious tint produced in colorless glass and white enamels 
by the presence of ferrous compounds. The latter greenish 
compounds are oxidized to the nearly colorless ferric salts while 
the slight pink tint imparted by the manganese still further 
counteracts the bluish color. The latter effect seems to be the 
more important, as red lead and other oxidizing agents do not 
have this decolorizing power. In a very finely divided state or 
in thin layers, manganese dioxide has a purplish-red color. 
The purple color of amethyst is due to a trace of MnOz2 or Mn;0, 
as Impurity in quartz crystal. 


OTHER Hyprous OxIDES orf MANGANESE 


Manganous Oxide.—By adding alkali hydroxide to a solution 
of manganous salt, white manganous oxide precipitates in a highly 
gelatinous form. The hydrous oxide adsorbs chloride slightly 
and sulfate strongly, so that the former is not carried down in 
the presence of the latter. Manganous hydroxide can be obtained ~ 
in regular hexagonal prisms similar to the mineral pyrochroite, by 
adding manganous chloride to a boiled concentrated solution of 
potassium hydroxide in an atmosphere of hydrogen.? By heating 
to 160°, all the oxide is carried into solution from which it precipi- 
tates in transparent crystals having a reddish tint. When 
pure, the crystalline hydroxide is fairly stable in the air, but if 
it contains even a small amount of alkali, it oxidizes very readily. 
Like magnesium hydroxide, it dissolves in excess ammonium 
chloride.* , 

A sol of hydrous MnO is formed by treating a solution of a 
manganous salt with alkali in the presence of protective colloids 


1 PatTren: J. Am. Chem. Soc., 25, 192 (1903). 
2 Dr ScHULTEN: Compt. rend., 105, 1265 (1887). 
3 Cf. Herz: Z, anorg. Chem., 21, 242 (1899); 22, 279 (1900). 


THE HYDROUS OXIDES OF MANGANESE 303 


such as gelatin,! Paal’s ‘‘sodium protalbinate,’’? albumin,? 
and nuclein acid. Because of its fine state of subdivision, it 
oxidizes readily to dioxide. In the presence of certain reducing 
agents such as hydroquinone and gallic acid, hydrous MnO, 
will give up oxygen, again forming colloidal MnO,° the process 
coming to a standstill only when there is no further reduction to 
MnO by the reducing agent, that is, when the oxidation of the 
added substance is complete. This action as an oxidase or 
oxygen carrier probably accounts for the rapid drying of manga- 
nese oxide paints, varnishes, and siccatives. The drying oil, 
such as linseed, doubtless plays the double role of protective 
colloid and of oxygen-consuming reducing agent.°® 

Manganic Oxide-—Hydrous manganic oxide is best prepared 
by hydrolysis of manganic salts, but it is also obtained in a 
more or less pure condition by the partial oxidation of manganous 
salts. By drying at 100°, the composition is said to be repre- 
sented by the formula Mn.O; - H.O, corresponding to the mineral 
manganite.’ When formed by the hydrolysis of potassium man- 
ganic cyanide, it is a black gelatinous mass which becomes less 
hydrous on heating with the mother liquor, changing in color 
from black to brown.’ Meyer® suggests that the color change 
may be due either to the decomposition of a hydrate or to a 
change in the size and physical character of the particles; he 
leaves the matter for someone else to decide. 

Mangano-manganic Oxide.—The oxide Mn;0, or MnO, -- 
2Mn0O is the most stable of all the oxides of manganese when 
heated in the air. Accordingly, higher oxides decompose around 

1Lopry DE Bruyn: Z. physik. Chem., 29, 562 (1898); Rec. trav. chim., 
19, 236 (1900). 

2 Kalle and Company: German Patent 180729 (1901). , 

3 TRILLAT: Bull. soc. chim., (8) 31, 811 (1904); Compt. rend., 138, 274 
(1904). 

4 SarAson: German Patent 272386 (1913). 

5 BERTRAND: Compt. rend., 124, 1032, 1855 (1897); Vinurers: Jbid., 124, 
1349 (1897). 

6 Cf. Levacue: Compt. rend., 97, 1311 (1883); 124, 1520 (1897). 

7 FRANKE: J. prakt. Chem., (2) 36, 31, 451 (1887); Mrymr: Z. anorg. Chem., 
81, 385 (1913). 

§ BERTHIER: Ann. chim. phys., (2) 20, 344 (1822); Hermann: Pogg. Ann., 
74, 303 (1848); Gorcru: Compt. rend., 106, 948 (1888). 

9Z. anorg. Chem., 81, 400 (1913). 


304 THE HYDROUS OXIDES 


940° and lower oxides oxidize in the air, forming Mn;0,. It is 
obtained in a hydrous condition more or less impure, by treating a 
mixture of manganous and manganic salts with alkali or by the 
oxidation of an ammonical solution of a manganous salt with 
oxygen. Christensen! obtained it by adding hydrous MnO, 
in small amounts at a time to a dilute solution of manganous 
chloride containing an excess of ammonium chloride. Depending 
on the exact condition of formation, the precipitate is yellow 
brown, red brown, to chocolate brown in color. 


1Z. anorg. Chem., 27, 321 (1901). 


CHAPTER XIV 
THE HYDROUS OXIDES OF THE PLATINUM FAMILY 


The platinum family consists of two groups of three closely 
related elements following iron, nickel, and cobalt in the eighth 
group of the periodic table. The metals of the first group— 
ruthenium, rhodium, and palladium—have atomic weights near 
100 and densities near 12; and the metals of the second group— 
osmium, iridium, and platinum—have atomic weights near 200 
and densities near 21. From their position in the periodic 
table, it is not surprising that these elements should form a 
number of compounds with oxygen. Some of the oxides can 
be obtained only in the anhydrous state while others may be 
precipitated as flocculent or gelatinous masses containing varying 
proportions of adsorbed water. ‘The oxides of the several ele- 
ments will be taken up in order, beginning with ruthenium. 


Hyprovus OxipEs oF RUTHENIUM 


Ruthenium Monoxide.—Hydrous RuO is thrown down by 
alkali from the blue solution of RuCl,. The precipitate is very 
highly dispersed and the adsorbed alkali cannot be washed out 
without peptization taking place. Moreover, like hydrous fer- 
rous oxide, the gel oxidizes very readily in the air. There seems 
little doubt of the chemical individuality of hydrous RuO, since 
the freshly precipitated oxide dissolves in hydrochloric acid, 
re-forming the characteristic blue solution of RuCle.! 

Ruthenium Sesquioxide.—Hydrous Ru,O3; is precipitated by 
adding alkali to a solution of the corresponding chloride.? 
Prepared in this way, the oxide contains adsorbed alkali that 


1 Remy: Z. anorg. Chem., 126, 185 (1923); cf., however, GuTBreR and 
RansouorFr: [bid., 45, 243 (1905). 
2Ciaus: Liebig’s Ann. Chem., 59, 234 (1846); Jory: Compt. rend., 114, 
291 (1892); Brizarp: Ann. chim. phys., (7) 21, 311 (1900); GuTsinr and 
Ransouorr; Z, anorg, Chem., 45, 253 (1905). 
305 


306 THE HYDROUS OXIDES 


cannot be removed by prolonged washing.! Krauss and Kiiken- 
thal? obtained a chloride- and alkali-free product by evaporating 
the hydrochloric acid solution of RuCl; to dryness, redissolving 
in water, and adding just enough potassium hydroxide to the 
dark-brown solution to cause complete precipitation of the oxide 
while the colorless liquid is still acid. The black flocculent pre- 
cipitate analyzed for trihydrate or hydroxide, after washing 
thoroughly and drying in an atmosphere of nitrogen at 120°. 
From these observations alone, it is impossible to say whether 
the compound is a true hydrate or a hydrous oxide. Since the 
pure preparation is soluble in acids, it furnishes a good starting 
point for making the most common ruthenium salts. The 
reddish-yellow solution of RuCl; deposits, slowly on standing but 
quickly when warmed, a black very finely divided precipitate 
said to be ruthenium oxychloride. This reaction is so delicate 
that 1 part of the metal imparts a distinct ink-like color to 100,000 
parts of water. 

Ruthenium Dioxide.—Claus* prepared hydrous rahe 
dioxide which he formulated Ru(OH),:3H.2O, by evaporating a 
solution of Ru(SO,). with caustic potash. The precipitate is 
dark red in color and adsorbs alkali strongly. According to 
Gutbier, the hydrous oxide cannot be obtained pure since it goes 
into colloidal solution extremely readily on washing. The pure 
anhydrous oxide results on roasting Ru(SO,)2 in the air* or on 
burning the metal.> It crystallizes in small very hard tetragonal 
pyramids possessing a green metallic luster and a bluish or green- 
ish iridescence. 

Ruthenium Pentoxide.—Hydrous Ru2O; results, according to 
Remy,® when hydrous RuO is allowed to oxidize spontaneously; 
but its identity has not been established with absolute certainty. 
Debray and Joly’ prepared what they took to be Ru2O;-2H.2O by 
neutralizing an alkali ruthenate with nitric acid; but Gutbier 

1 GuTBIER: Z. anorg. Chem., 95, 185 (1916); 109, 206 (1920). 

2Z. anorg. Chem., 182, 315 (1923). 

3 Liebig’s Ann. Chem., 59, 234 (1846). 

4GutTBiER and Ransonorr: Z. anorg. Chem., 45, 243 (1905). 

5’ GuTBIER, Leucus, WIkSssMANN, and Marscu: Z. anorg. Chem., 96, 
182 (1916). 


6 Z. anorg. Chem., 126, 185 (1923). 
7 Compt. rend., 106, 328, 1494 (1888). 


THE HYDROUS OXIDES OF THE PLATINUM FAMILY 307 


and Ransohoff! showed the alleged compound to be a mixture of 
hydrous Ru2O3 with some higher oxide. 

Salts containing hexavalent and heptavalent ruthenium are 
known, but the oxides RuO; and Ru,.O; have not been prepared. 

Ruthenium Tetroxide.—RuO, is not obtained in the hydrous 
state, but the anhydrous oxide is formed by passing chlorine into 
a solution of sodium ruthenate prepared by fusing ruthenium 
with sodium peroxide.? The heat of the reaction is sufficient 
to bring about the distillation of the oxide. The golden-yellow 
crystalline compound is fairly stable when dry, but decomposes 
quickly when moist. It dissolves slowly in water, giving a 
solution that is fairly stable, provided some free chlorine or 
hypochlorite is present. The oxide blackens organic matter 
readily and is reduced immediately by alcoholic potash with 
the separation of finely divided ruthenium. Serious explosions 
may occur if the solid oxide is treated with alcohol even in dilute 
solution. ? 


Hyprovus OxIpEs or RHODIUM 


Rhodium Sesquioxide.—Hydrous Rh.O; is precipitated as a 
black gelatinous mass by adding excess potassium hydroxide and 
a little alcohol to a solution of Na;RhCl,. If an excess of potash 
is not used, a sol is formed which deposits thin citron-yellow 
particles, said to be Rh(OH);-H.O.4 The gelatinous oxide is 
soluble in acids, forming the corresponding rhodium salts. 
Anhydrous Ru2O; only is formed by heating the finely divided 
metal in the air to 600 to 1000°. According to Gutbier,® the 
alleged formation of RhO* in this way was the result of incomplete 
combustion. 

Rhodium Dioxide.—Hydrous RhOz separates as a green pow- 
’ der when chlorine is passed into a solution of Rh.O3; in alkali. 
If the flow of chlorine is continued, the green precipitate dis- 


1Z. anorg. Chem., 45, 243 (1905); 95, 177 (1916). 

2 Myuius: Ber., 31, 3187 (1898); Gursier: Z. angew. Chem., 22, 487 
(1909). 

3 Desray and Joty: Compt. rend., 118, 693 (1891). 

4Cuaus: J. prakt. Chem., 76, 24 (1859); 80, 282 (1860); 85, 129 (1862). 

5 Z. anorg. Chem., 95, 225 (1916). 

6 Wim: Ber., 15, 2225 (1882). 


308 THE HYDROUS OXIDES 


solves, giving a deep-blue solution resembling the ammoniacal 
copper solution. The blue color is attributed to the alkali salt 
of rhodous acid, H2RhO,,' which decomposes gradually, precip- 
itating hydrous RhO2. 


Hyprovus OxipEs or PALLADIUM 


Palladium Monoxide.—The addition of sodium carbonate to 
a solution of palladous salt precipitates hydrous PdO as a dark- 
brown mass.2. When thrown down in the cold, the oxide is 
readily soluble in alkalies, but it loses this property when dried 
or when precipitated from boiling solution, owing to coalescence 
of the particles. The hydrous oxide is best obtained pure by 
hydrolysis of palladous nitrate. A PdO sol in paraffin oil has 
been introduced as a therapeutic agent under the name ‘‘Lep- 
tynol.’”’? The oxide serves as a catalyst for the reduction of 
aldehydes to alcohols.+* 

Palladium Sesquioxide.—Hydrous Pd:2O; is best prepared by 
the electrolytic oxidation of a concentrated solution of palladous 
nitrate at 8° with a current density of 0.5 ampere per square centi- 
meter. If the electrolysis is prolonged, hydrous PdO, is formed. 
This is not a direct oxidation, but the sesquioxide decomposes 
into dioxide and monoxide, the latter dissolving in the free acid 
and undergoing further oxidation.> The sesquioxide is formed 
also by the action of ozone on palladous nitrate. It is chocolate 
brown in color when first prepared, but on washing, it gets darker 
owing to agglomeration of the particles and loss of adsorbed 
water. 

Palladium Dioxide.—An impure hydrous PdO, is precipitated 
on adding caustic soda to a solution of KePdCls. As mentioned 
above, it is obtained free from alkali and basic salt by the anodic 
oxidation of the nitrate, but is not quite free from hydrous PdO. 
The fresh oxide is soluble in acids, but like the monoxide, its 
reactivity decreases rapidly on standing. It cannot be dehy- 


1 ALVAREZ: Compt. rend., 140, 1841 (1905). 

2 WOHLER and KGnia: Z. anorg. Chem., 46, 323 (1905). 

’ THorPE: ‘Dictionary of Chemistry,” 5, 51 (1924). 

4 SHRINER and Apams: J. Am. Chem. Soc., 46, 1683 (1924). 
6 WOuLER and Martin: Z, anorg. Chem., 57, 398 (1908). 


THE HYDROUS OXIDES OF THE PLATINUM FAMILY 309 


drated,! as it decomposes at the ordinary temperature under an 
oxygen pressure of 80 atmospheres. It is, therefore, a vigorous 
oxidizing agent. 


Hyprovus OxIpES orf OSMIUM 


Osmium Monoxide and Sesquioxide.—Claus and Jacobi pre- 
pared hydrous OsO by the action of warm concentrated alkali on 
OsSO3 in an atmosphere of nitrogen. It is a blue-black precipi- 
tate which takes up oxygen very rapidly from the air. The same 
authors obtained hydrous Os,O3 as a brown-red precipitate on 
adding alkali to a solution of K;OsClg. 

Osmium Dioxide.—A very highly hydrous form of OsQOz is 
precipitated by the addition of alkali to K2OsCls, and by the 
action of alcohol or other reducing agent on an alkali osmate 
such as K,OsO,.2 The hydrous mass may be converted into a 
fine powder by prolonged heating on the water bath in contact 
with the mother liquor. If the gel is dried, it forms a horny body 
which loses water explosively and emits flashes of light when 
heated above 100°. The more granular oxide aged on the water 
bath becomes incandescent quietly at the glow temperature. 
It is obvious that the primary particles of the gel are extremely 
small, the coalescence on ignition causing a marked decrease in 
surface energy with the accompanying glow. ‘The gel isa typical 
hydrous oxide, the water content of which is determined by the 
conditions of drying. The compound formed by hydrolysis of 
K.OsO, in the presence of alcohol and hydrogen and by the 
action of sulfuric acid on K,OsO, is hydrous OsOe and not 
H,OsO,4, as claimed by Moraht and Wischin.* 

Since the gel formed by reduction of alkali osmates contains 
such small primary particles, it can be peptized by shaking with 
an excess of water‘ or by treating with a small amount of alkali 


1 WOuLER and Konia: Z. anorg. Chem., 46, 323 (1905); 48, 203 (1906); 
57, 398 (1908); Bexuucct: Gazz. chim. ital., 35, I, 343 (1905); Z. anorg. 
Chem., 47, 287 (1906). 

2 Rurr and BorRNEMANN: Z. anorg. Chem., 65, 429 (1910); Rurr and 
Ratussura: Ber., 50, 484 (1917). 

3 Z. anorg. Chem., 3, 153 (1893). 

4Cxiaus and Jacost: J. prakt. Chem., 90, 65 (1863). 


310 THE HYDROUS OXIDES 


or ammonia.! Freundlich and Baerwind? dissolved 1 gram of 
OsO, in 50 cubic centimeters of water, added 10 cubic centimeters 
of ethyl alcohol and allowed the mixture to stand 24 hours. 
The precipitate of the dark dioxide was washed with alcohol and 
then peptized by shaking for several days with 800 cubic centi- 
meters of water. The deep-blue-black sol is fairly stable, but its 
stability is greater in the presence of a little alcohol or protective 
colloid. The particles are negatively charged and are not 
spherical, as they appear alternately bright and dark when viewed 
with a cardioid ultramicroscope. 

Osmium Tetroxide.—OsQO,, erroneously called osmic acid, 
does not form a hydrous oxide. It is obtained in transparent 
glistening needles by burning the metal or by the action of 
oxidizing agents on the lower oxides. It dissolves readily in 
water, forming a colorless liquid possessing a caustic or burning 
taste. The solution is used for staining biological preparations 
and also for taking finger prints,* the oxide being reduced to metal. 
The fumes of the oxide are very poisonous, attacking the lungs 
and eyes. It also acts violently on the skin causing painful 
wounds.® It may be employed as a catalyst for many oxidation 
reactions, ® 


Hyprovus OxIpEs OF [RIDIUM 


Iridium Sesquioxide.—Hydrous Ir.O; is obtained in much the 
same way as the corresponding rhodium compound which it 
resembles closely. When a solution of IrCl; - 6NaCl - 24H2O is 
heated with alkali in a stream of carbon dioxide, an impure 
hydrous IreO; separates that is greenish white to black in color, 
depending on the alkali concentration. The light-colored prod- 
ucts come down from dilute alkali, while excess alkali gives 


1 RurFr and Ratusspura: Ber., 50, 484 (1917). 

2 Kolloid-Z., 33, 275 (1923). 

3 Castro: Z. anorg. Chem., 41, 126 (1904); Paat and AmpErceR: Ber., 
40, 1392 (1907); 49, 557 (1916); AmperGErR: Kolloid-Z., 17, 47 (1915). 

4 DEVILLE and Drsray: Ann. chim. phys. (3) 56, 400 (1859); Compt. rend., 
78, 1509 (1874). 

5 MITCHELL: Analyst, 45, 125 (1920). 

6 HormMann: Ber., 45, 3329 (1913); Hormann, EHRHART, and SCHNEIDER: 
Ibid., 46, 1657 (1913). 


THE HYDROUS OXIDES OF THE PLATINUM FAMILY 311 


the black oxide containing relatively little water. This recalls 
the behavior of cupric oxide which dehydrates and darkens very 
quickly in the presence of excess alkali. Wohler and Witzmann! 
peptized the green oxide in dilute hydrochloric and sulfuric 
acids; concentrated acids dissolve it, giving reddish-yellow salts. 

Iridium Dioxide.—Hydrous IrQ,: is best prepared by adding 
alkali to a hot solution of NasIrCle, the sesquioxide first formed 
being oxidized to dioxide in a current of oxygen.? The fresh 
preparation is fairly soluble in acids and alkalies but it loses this 
property on drying.* The oxide can be gotten almost pure by 
drying the hydrous mass at 400° in carbon dioxide and then boil- 
ing with alkali and subsequently with sulfuric acid. 

The color of the oxide varies from light blue to black, depend- 
ing on the size of the particles and the structure and water content 
of the mass. Like the sesquioxide the precipitate is darker and 
less hydrous when it comes down from strong alkali solution. 

The solution obtained by the action of alkali on NageIrCl, 
in the cold has a violet color and contains hydrous IrQ, in sus- 
pension; after a time a violet modification of the oxide separates, 
which becomes blue on drying. Boiling the violet sol changes it 
to blue, probably owing to coalescence of the positively charged 
particles.4 Dilute hydrochloric acid peptizes the desiccator- 
dried preparation, giving a blue sol.® 

Iridium trioxide, IrO;, formed by fusing finely divided iridium 
with sodium peroxide or by the anodic oxidation of hydrous 
IrOz, is too instable to be isolated. 


Hyprovus OxIpEs oF PLATINUM 


Platinum Monoxide.—The black precipitate of hydrous 
PtO thrown down from PtCl. with caustic alkali cannot be 
washed free from chloride or alkali.* It is prepared in the pure 
state by adding the calculated amount of dilute caustic soda to 


1Z. anorg. Chem., 57, 323 (1908). 

2 Cuaus: J. prakt. Chem., 39, 104 (1846). 

3 Jory and Lrempe: Compt. rend., 120, 1341 (1895). 

4 WOuLER and WiTzMANn: Z. anorg. Chem., 57, 323 (1908). 
5 Cf. also PaAL, BIEHLER, and StryYeER: Ber., 60, 722 (1917). 
6 Limpia: Pogg. Ann., 17, 108 (1829). 


312 THE HYDROUS OXIDES 


a solution of K.PtCly.! As the fresh oxide takes up oxygen 
readily from the air, the precipitation, washing, and drying must 
be carried out in an atmosphere of carbon dioxide. When newly 
formed, it is readily soluble in dilute halogen acids but is insoluble 
in bases and in oxy acids other than sulfurous. Dried in a vacuum 
desiccator, the water content corresponds approximately to the 
formula PtO-2H.O. It holds on to its water very strongly, 
one sample retaining 6.6 per cent water after heating several days 
at 400°. PtO is a stronger oxidizing agent than the ees and a 
stronger reducing agent than the metal. 

Platinum Sesquioxide.— Wohler and Martin? obtained hydrous 
Pt.O; for the first time in a pure condition by adding solid PtCl; 
to a solution of sodium carbonate or by dissolving the chloride in 
concentrated potassium hydroxide and precipitating with acetic 
acid. The latter method yields a product containing some 
PtO.. The precipitate obtained at room temperature is light 
brown in color and highly hydrous; by boiling with alkali, it 
becomes less hydrous and darker; the dried preparation is almost 
black. The freshly formed oxide is not oxidized by boiling with 
water through which a stream of oxygen is passed; but it cannot 
be dehydrated completely without decomposition taking place. 
In chemical behavior it occupies an intermediate position between 
hydrous PtO and PtOs. 

Platinum Dioxide.—Wohler* prepared pure hydrous PtO:, by 
boiling a solution of platinic chloride with caustic potash, which 
converts PtClh’’ to Pt(OH).”. When cold, this solution is 
neutralized with acetic acid, and the hydrous oxide is obtained 
as an almost white precipitate which becomes yellow on drying. 
Even when dried in the air, the water content is less than cor- 
responds to the tetrahydrate PtO.-4H.O or H.Pt(OH)..4 It 
loses water continuously by lowering the vapor pressure of the 
surroundings or by raising the temperature; and there is no 
evidence of the existence of any definite hydrate. The last 2.5 


1 THOMSEN: J. prakt. Chem., (2) 15, 299 (1877); WOuuER: Z. anorg. Chem., 
40, 456 (1904); W6uuErR and Frey: Z. Elektrochem., 15, 133 (1905). 

* Ber., 42, 3958 (1909); cf., however, DupLEy: Am. Chem. J., 28, 59 
(1902); BLonpEL: Ann. chim. phys., (8) 6, 111 (1905). 

3Z. anorg. Chem., 40, 434 (1904); TopsGn: Ber., 3, 462 (1870). 

4 BeELLucctr: Atti accad. Lincei, (5) 12, 635 (1903). 


THE HYDROUS OXIDES OF THE PLATINUM FAMILY 3138 


per cent of water cannot be removed without decomposing the 
oxide. ‘The freshly precipitated product is soluble in acids and 
alkalies; but the thoroughly dried substance is insoluble in all 
dilute and concentrated acids with the exception of hydrochloric 
and aqua regia. It is sometimes called platinic acid, since its 
reactions with alkalies yields platinates such as K.Pt(OH). 
isomorphous with the stannates.! 

Platinum Trioxide.—By the electrolysis of a solution of hydrous 
PtO, in 2 N potassium hydroxide, a brilliant golden-yellow body 
of the composition K.O-3PtO; separates at the anode. When 
this is treated with dilute acetic acid in the cold, it yields the 
trioxide PtO3, a reddish-brown substance which loses oxygen 
readily and evolves chlorine slowly from dilute hydrochloric acid.’ 


1 BeLLuccr and PARRAVANO: Atti accad. Lincei, (5) 14, 459 (1905). 
2 WoOuLER and Martin: Ber., 42, 3326 (1909). 


CHAPTER XV 
TANNING 


Tanning is the process whereby the skin or hide of animals is 
converted into leather. Before subjecting hides to the tanning 
process, they must be treated to remove the hair, epidermis, and 
fat and to get the remainder of the substance in suitable condition 
to take up the tanning agent. The dermis or leather-producing 
portion of the skin consists essentially of bundles of fine connec- 
tive-tissue fibers about 1uin diameter, bound together irregularly. 
The fibrils consist essentially of a protein material, collagen, 
which is converted into gelatin by boiling with water. 

To prepare the hide for tanning, it is first immersed in lime 
water to which is usually added sodium sulfide to “‘sharpen”’ 
or hasten the action of the lime. The liming process not only 
removes the hair and destroys the epidermis, but it swells the 
collagen fibers and removes the cementing material between them, 
thereby splitting the bundles into their constituent fibrils. 
Following the liming process, the alkali is neutralized with dilute 
acids, and the hide is subsequently ‘‘bated”’ by subjecting it to 
the action of tryptic ferments in conjunction with ammonium 
chloride to remove the last trace of lime. The enzymes digest 
off part of the remaining connecting and epidermal substance, 
and completely emulsify the fat. This digestive action is stopped 
by ‘‘drenching,”’ that is, by treating the hide with fermenting 
bran infusions which bring the skins to a slightly acid state in 
which tryptic ferments are not active. The starchy matters of 
the bran are first converted into glucose, which undergoes bac- 
terial fermentation by several types of lactic-, butyric-, and 
acetic-acid-forming bacteria. As these bacteria develop only 
in solutions of feeble acidity and are destroyed by the accumula- 
tion of their own acid products, the acidity of the drench is auto- 
matically self-regulating and tends to produce a very slight acid - 
swelling of the skins. For chrome tanning, a similar result is 

314 


TANNING 315 


brought about by “pickling” the limed or bated skins in a bath 
consisting of a solution of sodium chloride and sulfuric acid in 
amount depending on the degree of basicity of the chrome liquor 
employed. After all undesirable impurities are removed and 
the collagen fibrils are brought to a flaccid slightly swollen condi- 
tion, the hide is ready for the tannage proper. In general, if 
the skin is soaked in infusions of barks, fruits, or galls which 
contain members of the class of compounds known as tannins, 
the process is called vegetable tanning; and if the tanning liquor 
is a mineral salt, it is known as mineraltanning. A consideration 
of the mechanism of these two processes will be taken up in order, 
even though any discussion of vegetable tanning might appear 
to be without the scope of this book. 


VEGETABLE TANNING 


It has been known for centuries that skin substance undergoes 
a marked change in properties when brought in contact with 
vegetable infusions, the active principle in which is now known 
to be tannin. Seguin regarded the tanning process as a reac- 
tion between hide, a base, and the tanning agent, an acid, giving 
leather, a salt. Berzelius and Dumas likewise considered leather 
to be a compound of hide and tanning agent without going into 
the mechanism of the process. Knapp,! whom Procter calls 
the father of leather chemistry, was the first to reason that 
leather could not be a definite »chemical compound since the 
amount of material taken up by the hide is not in any definite 
stoichiometrical proportion but depends on the concentration of 
the tan liquor. Moreover, since so many chemically different 
substances—tannin, alum, chromic sulfate, formaldehyde, stearic 
acid, etc.—can be used for tanning, Knapp concludes that the 
process must be essentially a physical one: “The two active 
substances are rendered insoluble in water by means of surface 
attraction (adsorption).’”’ The essential difference between hide 
and leather recognized by Knapp is that, in the latter, the 
fibers are no longer in the condition of a colloidal jelly, but may | 
be dried without adhesion, the substance remaining porous and 
flexible. 


1 Dinglers polytech. J., 149, 305 (1859), 


316 THE HYDROUS OXIDES 


Stiasny! was first to call attention to the similarity in the 
adsorbing capacity of carbon and hide powder. ‘Thus, both 
adsorb a wide variety of different substances; aromatic acids are 
adsorbed more strongly by both than are aliphatic acids; and with 
both adsorbents, acetic acid and the chloracetic acids possess 
approximately the same adsorbability in spite of their difference 
in strength. From such observations, Stiasny concludes that 
the taking up of tannin by hide powder is an adsorption process. 
Herzog and Adler? reached a similar conclusion from observa- 
tions of adsorption by hide powder of such substances as resorcin 
and pyrogallol, which are closely related to tannin but differ 
from the latter in forming molecular solutions in water. 

It remained for von Schréder* to demonstrate the adsorption 
of tannin from colloidal solution by carbon and hydrous alumina, 
as well as by gelatin and hide powder. Since the bacteria present, 
in hide powder cause decomposition of tannin, giving gallic acid, 
more consistent results can be obtained by sterilizing the adsorb- 
ent. Within the first hour after the adsorption, there exists an 
adsorption equilibrium between the tannin and the adsorbent, 
but the amount of tannin that can be removed by washing 
decreases with the time of standing. The effect of acids on the 
adsorption of tannin is less marked than that of alkalies. Thus 
0.05 N (NH,4)2CO3 added to a tannin solution cut down the 
adsorption, expressed in millimols per gram of adsorbent, from 
700 to 120. 

Since the collagen of hide is converted into gelatin by boiling, 
von Schréder compared the adsorption of tannin by hide powder 
to that of gelatin. The negatively charged particles of tannin sol 
are adsorbed by and precipitate a slightly acid and, therefore, 
positively charged gelatin sol. In slightly alkaline solution the 
gelatin particles are negatively charged, and adsorption by tannin 
with the accompanying mutual precipitation does not take place. 
As has been mentioned, hide powder in very dilute alkali likewise 
adsorbs tannin but slightly. Under comparable conditions, the 
adsorption capacity of gelatin and hide powder for tannin is 
very similar. As might be expected, a longer time is required for 

1 Kolloid-Z., 2, 257 (1908); Collegium, 118 (1908). 

2 Kolloid-Z., 2, 2d Supplement, III (1908). 

3 Kolloidchem. Bethefte, 1, 1 (1909). 


TANNING rah 


attaining the maximum adsorption with hide powder-in mass 

than with gelatin in the sol form where adsorption and mutual 

precipitation is quite rapid. Just as with hide powder, there is 

at first an adsorption equilibrium between gelatin and tannin, 

but this gradually gives way to an irreversible change in the mass. 
Von Schréder’s observations led him to say: 


Tanning with tannin is characterized by adsorption of the tanning 
agent. However, the adsorption compound is not leather at first; 
but this results in the course of time by a change in the adsorption com- 
pound whereby the tannin is more firmly held . 

Considering what has been said concerning the precipitation of 
gelatin by tannin and the parallelism in the behavior of gelatin and 
hide powder, one reaches the conclusion that the adsorption of tannin 
by hide powder is a concealed colloidal precipitation. Before the hide 
powder can absorb tannin, it must obviously be brought by swelling 
to such a condition that it can be precipitated. 


Von Schréder thus comes out definitely in support of the view 
that the first step in the tanning process is the mutual colloidal 
precipitation of negatively charged tannin and positively charged 
hide substance. It is not obvious just wherein the Procter- 
Wilson theory of vegetable tanning differs from von Schréder’s. 
It seems that Procter,! Wilson,? and others* were not aware of 
the definiteness of von Schérder’s viewpoint, for Procter writes 
in 1924: 


Knapp’s theory of the purely physical nature of the combination in 
tanning has remained the popular one in Germany, where it has been 
strongly supported by Wolfgang Ostwald and others, and considered 
as a case of “adsorption,” whatever that may be, but a view has 
gained ground in America and England that the change is of a colloidal 
character, and dependent on the opposite electric charges of the hide 
fiber and the tannin particles, which combine and electrically neutralize 
each other. Recent investigations by R. J. Browne, at the Procter 
Research Laboratory at Leeds, go far to prove that all vegetable tan- 
nins are colloidal in character, since they can be entirely removed from 
solution by ultrafiltration, and it is known from cataphoresis experi- 
ments that they are negatively charged, while hide fiber on the acid 


1 Bogue’s ‘‘Colloidal Behavior,” 2, 728 (1924). 
2 “The Chemistry of Leather Manufacture,” Easton, 271 (1923), 
3 THomas and Frrepen: J. Ind. Eng. Chem., 15, 839(1923), 


318 THE HYDROUS OXIDES 


side of its isoelectric point has a positive charge in consequence of the 
Donnan equilibrium. If two colloid suspensions of opposite charges 
come in contact, they combine, much as two oppositely charged ions 
would do, and, if mixed in the right proportions for complete neutral- 
ization, the precipitation is complete. The Procter-Wilson theory of 
tannage holds that leather is such a combination, and with regard to 
what may be called the first stage of vegetable tannage, there is strong 
evidence in its favor. Tannage only takes place when the hide fiber 
is slightly swollen with acid and so possesses a positive Donnan charge, 
and this charge will naturally vary with the difference between the 
hydrogen ion concentrations of the pelt and the liquor which is in 
equilibrium with it, which is greatest when the acidity is very small. 
In alkaline liquors, tannage does not take place. 


So far as I can make out, von Schréder’s interpretation of the 
mechanism of the first step in the tanning process is the same as 
Procter and Wilson’s. What the latter have done in addition is 
to attempt to give the origin of the charge on the collagen of 
the hide. Thus it is assumed that in equilibrium with a tan 
liquor having a pH value lying in the range 2 to 5, collagen 
(represented by C) forms a compound, CHA, which is completely 
ionized into the positive ion CH’ and the negative ion A’. Since 
the hypothetical collagen cations are a part of an elastic structure 
which cannot diffuse, the conditions necessary for a Donnan 
equilibrium obtain.t When equilibrium is attained between the 
collagen and the acid: 


In the tan liquor let v= [Ht Ae 
and in the collagen jelly let y = [H’] 

and 2 =r 

from which [A] =yt+e 


The equation of products may now be written: 
a? = y(y + 2) 


in which the product of equals is equated to the product of 
unequals. It follows, therefore, that the sum of the unequals is 
greater than the sum of the equals, or that 2y + 2, the sum of the 
diffusible ions in the hide jelly, is greater than 22, the sum of 


1Cf. Chap. I, p. 18, 


TANNING 319 


the ions in the tan liquor. This gives rise to an electrical differ- 
ence of potential between the jelly phase and the external solu- 
tion, which may be formulated thus: 
Pie, RT —2+ V4e?+2? 

22 


E = —,~ log. steal log. 





By similar reasoning the electrical difference of potential H, 
between the surface film of the tannin particles and bulk of the 
solution is given by 





hes KT log, = KT log. BE a8 an 
Yi F ay pate Bae a 
where 2; is the concentration of the cations balancing the negative 
charge on the tannin particles and y; the concentration of the 
anions [A’] in the surface film. Since # and E, are of opposite 
sign, the Procter-Wilson theory assumes that the first step in 
the mechanism of tanning results from the tendency for E and EL, 
to neutralize each other. 

The equation for the difference of potential between a positive 
collagen jelly and the surrounding solution is deduced from the 
specific assumption that hydrochloric acid, say, combines with 
collagen, forming highly ionized collagen chloride in which the 
collagen is the constituent of a complex cation whose free diffusion 
is restricted. It should be emphasied that this does not furnish 
any proof whatsoever of the existence of a definite highly ionized 
compound, collagen chloride, yielding a collagen cation. One 
arrives at exactly the same equation by making the more probable 
- assumption that collagen, like the oxides of iron, chromium, and 
aluminum, adsorbs hydrogen ion more strongly than chloride ion 
and so possesses a positive charge in dilute hydrochloric acid 
solution. The hydrogen ion adsorbed by the jelly is not free to 
diffuse, thus imposing the constraint conditions necessary for a 
Donnan equilibrium. As Donnan! puts it: ‘An adsorption of 
hydrogen ions by colloidal aggregates or micelles (constituting 
the units of the molecular network) would lead to the same 
constraint conditions and the same general equations as the 
ionization of the amphoteric protein molecules assumed by 
Procter.” 


1 Chemical Rev., 1, 89 (1924), 


320 THE HYDROUS OXIDES 


The initial step in the tanning process would thus appear to 
be neutralization by adsorption of negative tannin by positive 
collagen, which owes its charge to preferential adsorption of 
hydrogen ion. The isoelectric point of collagen is claimed to 
be at pH = 5;! and the amount of tannin adsorbed by a given 
amount of hide powder increases with decreasing pH values, as 
would be expected. It caused considerable worry, however, to 
find an increase in the adsorption of tannin with increasing pH 
on the alkaline side of the alleged isoelectric point. This reaches 
a maximum around pH = 8, above which it falls off rapidly to 
zero.” A partial explanation of this was forthcoming when 
Wilson and Gallun’ found a second isoelectric point for collagen 
at pH = 7.7. Two tautomeric forms of collagen are, therefore, 
assumed to exist: one, C,, stable in acid solution with an iso- 
electric point at pH = 5; and a second, Cs, stable in alkaline 
solution with an isoelectric point at pH = 7.7. On increasing 
the pH value from 5 to 7.7, if the change from C, to Cy proceeds 
at a greater rate than positive C, changes to negative Ca, then 
the net result will be an increase in the amount of positive C, - 
and an increase in the amount of tannin adsorbed. But there is 
still some tannin adsorbed above pH = 7.7, more in commercial 
tanning extracts than in tannic acid. Thomas and Kelly® 
assume the existence of some complex organic reaction to account 
for this. 

While one cannot deny that the initial step in tanning by tannin 
may involve factors other than neutralization by adsorption of 
negatively charged tannin by positively charged hide, there 
seems no necessity, at least for the present, to postulate any 
other action to account for the fact that there appears to be some 
adsorption of tannin above pH = 7.7 and below pH = 2. 
For, if there are two modifications of collagen, Ca and Cy, with 
different isoelectric points in contact with each other, then each 
is certain to influence the other, and the values pH= 5 and 


1 Porter: J. Soc. Leather Trades’ Chem., 5, 259 (1921); 6, 83 (1922); 
Tuomas and Ketiy: J. Am. Chem. Soc., 44, 195 (1922). 

2 THomas and Ketty: J. Ind. Eng. Chem., 15, 1148 (1923). 

8 J. Ind. Eng. Chem., 16, 71 {1923)% 

4 THOMAS and Onin J. Ind. Eng. Chem., 16, 800 (1924). 

5 J. Ind. Eng. Chem., 16, 31 (1924). 


TANNING 21 


pH = 7.7 are not true isoelectric points at all; but each is an 
average or compromise value at which the mutual effect of two 
collagens of the same or opposite charge give a minimum. If 
the supposed Cy, could be isolated from the influence of the 
supposed Ca, the isoelectric point of the former might very well 
be at a higher value than pH = 7.7. 

Attention has been called to von Schréder’s observation that 
the adsorption of tannin by hide powder is completely reversible 
for a short time; but on standing, the process becomes irreversible. 
Not only can no tannin be extracted from the hide by water, but 
it resists the action of dilute alkalies; that is, leather is formed. 
Justin-Mueller! believes that the second stage in the tanning proc- 
ess following adsorption is some chemical reaction between tannin 
and hide. A similar view seems to be favored by Freundlich,? 
although he does not commit himself definitely. On the other 
hand, a number of people* come out squarely in support of the 
view that the process is physical throughout. Thus Moeller+ 
states ‘‘that the changes which tannin colloid undergoes after 
being taken up by hide substances were found to depend solely 
on irreversible colloidal changes of state. Simple chemical 
processes do not occur.”’ Moeller’ considers leather to be animal 
hide, the elementary particles of which are microcrystalline 
micelles protected from hydrolytic influences by a sheath of 
tan particles. 

Some attempt has been made by Meunier,’ Fahrion,’ and 
others to work out a purely chemical interpretation of vegetable 
tanning. Thus Meunier obtained a leather of remarkable per- 
manence by bringing skin in contact with hydroquinone. A por- 
tion of the quinone was reduced to quinol and Meunier concludes 


1 Kolloid-Z., 6, 40 (1910). 

2 “Blements of Colloid Chemistry,” translated by Burger, London, 186 
1925). 
3 Von Scurover: Kolloidchem. Bethefte, 1, 53 (1909); Strasny: Collegium, 
118 (1908); Kolloid-Z., 2, 257 (1908); 31, 299 (1922); GoLDMANN: Collegium, 
93 (1908). 

4 MoE.LuER: Collegium, 39 (1917). 

5 Z. Leder-Gerberei Chem., 1, 360 (1922). 

6 Chimie & industrie, 1, 71, 272 (1918); J. Am. Leather Chem. Assoc., 
13, 530 (1918); Meunrer and Sryewetz: Mon. sct., 28, 91 (1908). 

7Z. angew. Chem., 22, 2083, 2135, 2187 (1909). 


322 THE HYDROUS OXIDES 


that this reduction is accompanied by oxidation of the collagen, 
whereupon the oxidized collagen combines with the remaining 
quinone, giving leather. Meunier postulates the formation of 
quinones in vegetable tanning materials, which react with col- 
lagen to form leather. Powarnin!' objects to the assumption that 
the quinones result from oxidation and suggests that they are 
formed by a tautomeric change which, for tannin, is assumed to be: 


CH CH 
Yaa 4 
HC C — OH = HC CH — O 
| | as | | | 
HC C — OH HC CH — O 
Bee Soe 
CH CH 
Enol form Keto form 


The enol form is supposed to be stable in alkaline solution and 
the keto form in acid solution. Only the latter form is assumed 
to have tanning properties. As yet, these views of the tanning 
process lack definite experimental foundation;? but they indicate 
the probable existence of a chemical as well as a physical action 
in vegetable tanning. 

Formaldehyde has tanning properties’ in solutions having a 
pH value greater than 4.8, the best practical results being 
obtained between pH = 5.5 to 10.0.4 Meunier believes that a 
definite chemical compound is formed between the aldehyde and 
oxidized collagen; but this appears doubtful, as the formaldehyde 
can be recovered quantitatively from formaldehyde leather 
simply by digesting with dilute hydrochloric acid. 


MINERAL TANNING 


Any mineral salt may be employed for tanning leather, pro- 
vided it undergoes hydrolytic dissociation forming a colloidal 


1 Collegium, 634 (1914). 

2Cf., however, THomMas and Keuiy: J. Ind. Eng. Chem., 16, 800 
(1924); THomas and FostErR: J. Am. Chem. Soc., 48, 489 (1926). 

3 British Patent 2872 (1898). 

4 Hny: J. Soc. Leather Trades’ Chem., 6, 131 (1922); THomas and KELLy: 
J. Ind. Eng. Chem., 16, 925 (1924). 


TANNING 320d 


hydrous oxide or basic salt. Actually only the salts of iron, 
aluminum, and chromium have been employed, and of these the 
salts of chromium are by far the most important and so will be 
considered first. bee 

Chrome Tanning.—As early as 1858, Knapp! described a proc- 
ess for tanning hide with solutions of salts of aluminum, iron, 
and chromium; but the first successful method of mineral tanning 
was invented by Augustus Schultz in 1884. In Schultz’s two- 
bath process, the skins are treated with an acidified solution of 
potassium dichromate until the liquor penetrates them thoroughly 
after which they are put into a bath of acidified sodium thiosul- 
fate which reduces the chromate in the hide to chromic salt, the 
tanning agent. In 1893 Dennis revived and patented Knapp’s 
original single-bath tan liquor which consists of a partially neu- 
tralized solution of chromic chloride. Dennis prepared the bath 
by dissolving chromic oxide in hydrochloric acid and subsequently 
rendering this more basic by adding caustic soda. Later Procter? 
showed that good tanning liquors could be prepared by reducing 
bichromate solution with glucose in the presence of enough 
hydrochloric acid to leave the solution basic. Basic chromic 
sulfate was found to be superior to the chloride for one-bath 
tanning and is now almost universally employed.* A useful 
method of preparing a satisfactory bath consists in the reduction 
of a strong solution of sodium bichromate directly with sulfur 
dioxide. A concentrated liquor can be obtained in this way and 
diluted as required. In view of the relatively weak character of 
sulfurous acid, the liquor is sufficiently basic for many purposes. 
The equation for the reduction is usually written 


NaeCr2O7 -+- 350.2 + H.O = NaesO, + 2CrOHSO, 


This merely represents the relative basicity of the final liquor, 
but there is no assurance of the formation of a definite basic 
salt like that formulated. 


1 “Nature and Essential Character of the Tanning Process and of Leather,”’ 
J. G. Cotta Buchandlung (1858); English translation, J. Am. Leather 
Chem. Assoc., 16, 658 (1921). 

2 Leather Trades’ Rev., Jan. 12 (1897). 

3 Wiuson: “‘The Chemistry of Leather Manufacture,”’ Easton, 278 (1923). 

4 BatpERSTON: Shoe & Leather Rep., Oct. 18 (1917); Procter: J. Roy. 
Soc. Arts., 66, 747 (1918). 


324 THE HYDROUS OXIDES 


That basic liquor is more satisfactory for tanning is well illus- 
trated by some observations of Thomas, Baldwin, and Kelly! 
on the rate of taking up of chromic oxide by hide powder from a 
commercial tan liquor and from a solution of chromic sulfate. 
The chrome liquor had a basicity corresponding to the formula 
Cr(OH)1.2(804)o.9 and contained 17 grams Cr.O; per liter. 
The chromic sulfate contained 164 grams Cr.O; per liter. The 
results are shown in Fig, 20. The time in hours covered by 


Time, hours 








Cr,03 Adsorbed, milligrams per gram of hide substance 





Fic. 20.—Adsorption of chromic oxide by hide substance. 


the experiment with the commercial chrome liquor is plotted on the 
top horizontal axis and the time in days covered by the experi- 
ments with chromic sulfate solution on the bottom horizontal axis. 
The amount adsorbed is determined by direct analysis of the 
leather. It will be seen that the rate of tanning is very much 
less in the chromic sulfate solution which has a hydrogen ion con- 
centration about twenty times greater than the commercial 
liquor. Moreover, the amount of chromic oxide taken up from 


‘J. Am. Leather Chem. Assoc., 15, 147 (1920); THomas and Keuzy: J bid., 
15, 487 (1920). 


TANNING 325 


the commercial liquor has not reached a limiting value in 4 days; 
but is appreciably greater than the limiting value in the case of 
pure chromic sulfate. Since the amount of chromic oxide taken 
up and the velocity of the process is greater in the more basic 
solutions, it is obvious why such solutions are used in practice. 
The basicity must be subject to careful control, however, since 
if too basic, the bath is rendered turbid in the presence of hide, 
owing to precipitation of hydrous chromic oxide. This is of 
importance in connection with the theory of chrome tanning, 
which will next be considered. 

The most plausible theory of chrome tanning is that the hide 
fibrils adsorb from the tan liquor hydrous chromic oxide or basic 
salt which subsequently ages, giving a protective coating. This 
film not only keeps the fibrils separated and thereby prevents 
their coalescence on drying but protects them from the action 
of water and dilute alkali. As Rochelle salt dissolves even an 
aged hydrous chromic oxide, it is not surprising to learn that a 
chrome-tanned leather is detanned by soaking in a solution of 
this salt.' The detanned leather can be tanned once more by 
washing and soaking in fresh chrome liquor. 

Davison? determined the amount of chromic oxide taken up 
in 4 hours by a constant amount of hide powder, from various 
concentrations up to 1.5 grams Cr.O3; per liter, of a single-bath 
chrome-tanning solution. On plotting the chromic oxide taken 
up against the concentration of the residual solution a continuous 
curve is obtained which corresponds with the ordinary adsorption 
formula. This supports the view that the initial step in chrome 
tanning consists in adsorption of hydrous chromic oxide or basic 
_ salt.3 | 

Attempts have been made to interpret the chrome-tanning 
process as a mutual precipitation of oppositely charged particles 
just as in the case of vegetable tanning. The difficulty encoun- 
tered is that hydrous chromic oxide in acid solution takes a posi- 
tive charge just like the hide. Thompson and Atkin‘ suggest that 


1 Procter and Wixson: J. Soc. Chem. Ind., 35, 156 (1916). 

2 J. Phys. Chem., 21, 190 (1917). 

3 BENNETT: J. Soc. Leather Trades’ Chem., 1, 130, 169 (1917); cf., how- 
ever, Witson, Tuomas, et al.: J. Am. Leather Chem. Assoc., 12, 450 (1917). 
4 J. Soc. Leather Trades’ Chem., 6, 207 (1922). 


326 THE HYDROUS OXIDES 


the active constituent of the chrome-tan liquor is a negative ion 
or colloidal particle having a composition such as Cr(OH);- - 
CrOCl- Cl’ which combines with positively charged collagen, 
forming leather. This view was called in question by Seymour- 
Jones! who found that hide was tanned in a normal fashion in a 
basic chromic chloride solution which showed no anodic migration 
of chromium whatsoever. Seymour-Jones” also attempted the 
ultrafiltration of a typical tanning bath containing 270 grams 
chromic oxide per liter prepared by reduction of a solution of 
sodium bichromate with sulfur dioxide. The solution passed 
unchanged through a collodion disk ultrafilter and through filter 
papers impregnated with 1 and 5 per cent gelatin solutions, 
respectively, and subsequently hardened. This was taken to 
mean that colloidal chromic oxide or basic salt plays no réle in 
chrome tanning. There is, however, no doubt of the presence of 
colloidal chromic oxide in certain technical tan liquors. Thus, 
Wintgen and Lowenthal? ultrafiltered a so-called one-third basic 
commercial tan liquor prepared by mixing 20 grams chrome alum 
in 170 cubic centimeters of water with 7 grams of crystalline 
sodium carbonate in 20 cubic centimeters of water. Using a 
very thick fine hardened ultrafilter and applying a pressure of 75 
atmospheres, they obtained a filtrate consisting of chromium salt 
in molecular solution; and a residue possessing the appearance 
and properties of colloidal chromic oxide. It is altogether prob- 
able that Seymour-Jones could have ultrafiltered some colloidal 
chromic oxide from his tan liquor had he used a sufficiently dense 
filter. This is, however, more or less beside the point, as one can 
tan leather in a chromic salt solution containing but little colloidal 
oxide. 

If chromic sulfate is placed in solution, an equilibrium exists 
that may be represented thus: 


Cre(SO.4)s +- xH,.O —s Cr.03;7H.O fe 3H.SO, 
or, if preferred, by 
Cr2(SOx)s + 12H.O <= [Cr(OH)> . (H20) 4]2S04 ++ 2HSO, 


1J. Ind. Eng. Chem., 15, 265 (1923). 
2 J. Ind. Eng. Chem., 15, 75 (1923). 
3 Kolloid-Z., 34, 294 (1924), 


TANNING 327 


since Werner! has prepared a definite crystalline basic salt, insolu- 
ble in water, of the formula indicated. When hide is placed in 
such a solution, it adsorbs acid strongly, thus displacing the 
equilibrium to the right with the consequent precipitation of 
the insoluble hydrous oxide or basic salt on the surface of the 
particles of hide, where it is adsorbed. The amount deposited in 
the hide substance under these conditions is obviously small and 
so the tannage is relatively light. If, on the other hand, a por- 
tion of the acid is neutralized, the adsorption of acid by the hide 
brings about a correspondingly greater precipitation of hydrous 
oxide or basic salt, and the tannage is correspondingly heavy. 
As I have already pointed out, if the tan liquor is rendered too 
basic, the adsorption of acid by the hide causes precipitation of 
the hydrous oxide in the liquor rendering the latter cloudy. 
Obviously, a careful control of the conditions is necessary for 
successful tanning. In general, if the acidity is too high, the 
penetration is good, but the amount of chromic oxide deposited 
is slight; whereas if the basicity is too high, the bath contains 
hydrous chromic oxide in too coarse a state of subdivision to 
penetrate well. 

It should be emphasized that the displacement to the right of 
the hydrolytic decomposition of chromic salt is occasioned not 
only by adsorption of sulfuric acid but by adsorption of hydrous 
chromic oxide as well. To illustrate, let us consider the adsorp- 
tion phenomena which take place from solutions of chromate and 
dichromate with hydrous alumina. In such solutions, the follow- 
ing equilibrium exists: 


Cr.0 7” a H.O one PAS & oe 2CrO.” 


If a sample of highly purified “grown alumina’’? is added to a 
solution of red dichromate, the solution becomes yellow. This is 
because hydrous alumina adsorbs hydrogen ion, strongly shifting 
the equilibrium to the right. But the alumina also adsorbs 
chromate which can be determined quantitatively and can be 
detected qualitatively by the color it imparts to the adsorbent. — 
This likewise tends to displace the equilibrium to the right. 
1 Ber., 41, 3447 (1909). 


2 Pecanas: Z. angew. Chem., 18, 801 (1904); Kolloid-Z., 2d Supplement, 
XI (1908). 


328 THE HYDROUS OXIDES 


If, instead of adding powdered alumina to a solution of dichro- 
mate, one adds aluminasol stabilized by preferential adsorption of 
hydrogen ion, the adsorption capacity of the alumina for hydro- 
gen is partially supplied and the equilibrium is not disturbed 
appreciably, the solution remaining red. Under these conditions, 
the adsorption of chromate is relatively small; and incidentally, 
the amount of dichromate carried down is less than that for most 
multivalent ions.' In the same way, when hide is placed in a 
chromic sulfate solution containing a relatively large amount of 
hydrogen ion, hydrous chromic oxide or basic salt is adsorbed, 
as well as sulfuric acid; but the adsorption of the former is much 
greater from more basic solutions. 

After the tannage is complete, the skin is left in a somewhat 
acid condition. In practice, it is rendered nearly neutral by 
treating with a dilute alkaline solution. Even after this treat- 
ment chrome leather is characterized by having a relatively high 
sulfuric acid content. Only a trace is free at any one time, but 
as soon as this trace is removed, more is immediately liberated. 
A part of this sulfuric acid is adsorbed by the hide and a part by 
the hydrous oxide, while some may exist in solid solution in the 
hydrous oxide or as a basic salt. 

The addition of neutral salts to a bath cuts down the adsorption 
of chromic oxide by the hide. In the case of chlorides, this may 
be due to the observed increase in the hydrogen ion concentra- 
tion ;? but sulfates decrease the hydrogen ion concentration which 
should favor increased adsorption of chromic oxide. To get 
around this difficulty, Wilson and Gallun’ postulate the formation 
of addition compounds between the chromium compounds and 
the added salt, which are supposed to be endowed with the prop- 
erty of tanning less readily than the original chromium com- 
pounds. It is probable that a great deal of the effect of neutral 
salts is due to their adsorption by the hide, which cuts down the 
adsorption of hydrous chromic oxide. 


1 Weiser and Mippieton: J. Phys. Chem., 24, 647 (1920). 

2 Poma: Z. physik. Chem., 88, 671 (1914); HaRNEpD: J. Am. Chem. Soc., 
37, 2460 (1915); THomas and Batpwin: J. Am. Leather Chem. Assoc., 13, 
248 (1918); J. Am. Chem. Soc., 41, 1981 (1919); THomas and Fostsr: J. Ind. 
Eng. Chem., 14,132 (1922). 

° J. Am. Leather Chem. Assoc., 15, 273 (1920). 


TANNING 329 


A purely chemical theory of chrome tanning receives its most 
enthusiastic support from Wilson and his collaborators.! It is 
the opinion of these investigators that even in acid solution, 
there are some negatively charged groups in the collagen structure. 
In the tanning process, Cr(OH),.’ ions or ions of similar structure 
are supposed to diffuse into the jelly composing the hide and to 
attach themselves to negatively charged groups wherever 
encountered, giving salts that have been designated chromium 
collagenates. Attempts have been made to establish the exist- 
ence of such salts by Baldwin? and by Thomas and Kelly.® 
Baldwin studied the fixing of chromic oxide from various liquors 
containing 0.38 to 66.4 grams of chromic oxide per liter and found 
that the amount taken up reaches a maximum in a bath con- 
taining 15 to 20 grams per liter. Davison failed to observe this 
maximum, as he worked with lower concentrations of tan liquor. 
Thomas and Kelly repeated Baldwin’s experiments with con- 
centrations varying from 0.36 to 202 grams chromic oxide per 
liter, and confirmed the result that the amount of chromic oxide 
taken up per gram of hide powder in 48 hours reached a maximum 
in a solution containing approximately 16 grams of chromic 
oxide per liter, after which the curve sloped downward, reaching a 
minimum when the concentration of chromic oxide in solution 
was approximately 150 grams per liter as shown in the lower 
curve, Fig. 21; the experiments were repeated, keeping the liquor 
in contact with the hide for 8.5 months, with the results given in 
the upper curve, Fig. 21. The conclusions drawn from these 
observations are the following: Wilson* found that 750 grams of a 
certain collagen take up 1 mol of hydrochloric acid, forming what 
he believes to be collagen chloride, a salt of a weak monoacid base. 
He, therefore, assumes the combining weight of collagen to be 
750. Using this value, Thomas and Kelly calculate that 4 
equivalents of chromium are combined with 1 of collagen at the 
maximum in the 48-hour curve which represents a definite 
compound, tetrachrome collagen. In the 8.5-month run, the 


1 Witson: ‘‘The Chemistry of Leather Manufacturer,’ Easton, 278-308 
(1923). 

2 J. Am. Leather Chem. Assoc., 14, 433 (1919). 

3 J. Ind. Eng. Chem., 18, 65 (1921); 14, 621 (1922). 

4 J. Am. Leather Chem, Assoc., 12, 108 (1917). 


330 THE HYDROUS OXIDES 


maximum is approximately twice as high as in the 48-hour run, 
a circumstance that is claimed to prove the existence of octa- 
chrome collagen. The octachrome curve shows a slight bend 
at a higher concentration of chrome liquor where the fixation of 
chromic oxide is believed to be sufficiently near the theoretical 
for tetrachrome collagen to justify postulating its formation. 
After championing the use of thermodynamic formulas to 
interpret tanning by tannin, one wonders why-no attention what- 


en 
>) 
Oo 





250 


Cr,0, Adsorbed milligrams pergram of hide substance 


Concentration, grams Cr.03 per liter 


Fig. 21.—Effect of concentration of chrome liquor on the adsorption of 
chromic oxide by hide substance. 


soever seems to have been paid to the phase rule in interpreting 
the results with chrome tanning. In the light of this generaliza- 
tion, the curves obtained by Thomas and Kelly certainly do not 
offer convincing proof of the formation of chromium collagenates. 
On the contrary, they indicate that quite the opposite is true. 
A maximum is observed repeatedly in the taking up of one sub- 
stance from solution by another. For example, the lower curve 
in Fig. 22 shows the adsorption of acetic acid from toluene solu- 
tion by animal charcoal! and the upper curve, the adsorption of 


1 Scumipt-WaLTER: Kolloid-Z., 14, 242 (1914). 


TANNING | ax 


phenol from solution in ethyl alcohol by the same adsorbent.’ 
The maxima in these curves are no more indicative of compound 
formation than any other points on the curves. Freundlich’ 
observed maxima in the adsorption of strychnine nitrate from 










bed per Gram Carbon 


04 05 


Gram Solute per Gram of Solution 





Grams Adsor 


Fia. 22.—Adsorption by carbon of (1) phenol from ethyl alcohol and (2) acetic 
acid from toluene. 


aqueous solution by carbon, wool, and arsenious sulfide; and in 
the adsorption of crystal violet by carbon and fibers*. Slmilra 
observations were made by Biltz and Steiner* in the absorption 
of dyes, such as night blue and Victoria blue, by wool and car- 


1 Gusrarson: Z. physik. Chem., 91, 397 (1916). 

27. physik. Chem., 78, 400 (1910); FREUNDLICH and PosER: Kolloidchem. 
Beihefte, 6, 295 (1914). 

3 Freunpuicu and Lossv: Z. physik. Chem., 59, 284 (1907). 

4 Kolloid-Z., 7, 113 (1910). 


332 THE HYDROUS OXIDES 


bon. Dreyer and Douglas! found that the adsorption of agglu- 
tinin by bacteria reached a maximum at a certain concentration 
and thereafter decreased. ‘‘In short,” says Freundlich,? ‘‘by far 
the majority of the adsorption curves that are not entirely 
regular show a maximum in the adsorbed mass with increasing 
concentration, followed by a falling off until the adsorption is 
negative.? 

A number of cases have been reported where a change in the 
physical character of the adsorbent leads to a maximum in the 
adsorption curve. Thus Lottermoser and Rothe* observed a 
decrease in the adsorption of potassium iodide by silver iodide 
above a certain concentration of electrolyte. This was traced 
to a change in the structure of the silver iodide, which became 
denser and more granular. Freundlich and Schucht? noted the 
spontaneous transformation of amorphous mercuric sulfide to 
a crystalline form that shows a decreased power of adsorbing 
dyes. Wagner® showed that when salts of many of the hydrous 
oxides are hydrolyzed, they absord the free acid to some extent 
and later give it up owing to a change in the physical character 
of the adsorbent. While the chromic-oxide hide-powder curves 
are typical of adsorption curves showing a maximum, it is prob- 
able that the irreversible change in state which hide powder 
undergoes in contact with tanning liquor is in part responsible 
for this maximum. ‘The increasing hydrogen ion concentration 
with increasing concentration of tan liquor likewise contributes 
to the cutting down of the adsorption of chromic oxide at higher 
concentrations. The important thing is that the maximum in 
the continuous curves should not be construed as indicating the 
formation of definite chromium collagenates any more than any 
other point on the curve. At suitable points on the curve, a 
whole series of definite salts from monochrome to octachrome 
collagenate may be assumed to exist; but this does not indicate, 
let alone prove, their existence. 


1 Proc. Roy. Soc., 82B, 185 (1910). 

2 “ Kapillarchemie,”’ 246 (1922). 

3 Cf. WituiAMs: Med. fr. K. Vet. Akad. Nobelinst., (2) No. 27 (1913). 
4Z. physik. Chem., 62, 359 (1908). 

5 Z. physik. Chem., 86, 660 (1913). 

6 Monatshefte frir Chemie, 34, 95, 931 (1913). 


TANNING 333 


Alumina Tanning.—An alumina tan bath consists of basic 
aluminum sulfate together with enough sodium chloride to 
prevent undue swelling of the skin. ‘The hydrous oxide or basic 
salt appears to be adsorbed less strongly than in the case of 
chrome tanning, and the freshly treated hide cannot be washed 
without swelling. Moreover, hydrous chromic oxide ages much 
more rapidly than hydrous alumina, and so it is necessary to 
keep the alumina-treated skins in the dried state for weeks or 
months before a satisfactory leather is obtained. Even at best, 
alumina-tanned leather is not so permanent as chrome-tanned 
leather, probably because hydrous alumina becomes crystalline 
on ageing and so does not afford such good protection to the 
hide particles as does the amorphous film of hydrous chromic 
oxide which never assumes the crystalline form. 

Iron Tanning.—Ferric salts may be employed as tanning 
agents, but attempts to manufacture iron-tanned leathers have 
not met with success. According to Procter,! a part of the diffi- 
culty arises from the fact that ferric oxide acts as an oxygen 
carrier, causing slow oxidation of the hide and consequent deteri- 
oration. Moreover, difficulty is encountered in neutralizing 
the excess sulfuric acid after tanning. When the leather is 
treated with a dilute alkali solution, the adsorbed hydrous oxide 
is displaced, and any normal or basic salt is converted into col- 
loidal hydrous oxide and washed out of the skin.” Jackson and 
How? claim to have prepared a fairly good leather by adjusting 
the acidity so as to give a tan liquor in which the ratio of equiv- 
alents of hydroxide groups to equivalents of acid radical is 
never less than 1:5 nor more than 1:3. After tanning, the neu- 
tralization is effected very gradually. 

Silica Tanning.—Graham*‘ pointed out in 1862 that gelatin 
was precipitated by colloidal silica. The precipitate was insolu- 
ble in water and was not decomposed by washing; in other words, 
the gelatin was tanned. Hough® found that purified colloidal 
silica is much too instable to serve as a tanning agent. As would 


1 “The Principles of Leather Manufacture,” 2d ed., p. 275. 
2 JeTTMAR: Cuir, 8, 74, 106 (1919). 

3 J. Am. Leather Chem. Assoc., 16, 63, 139, 202, 229 (1921). 
4 J. Chem. Soc., 15, 246 (1862). 

5 Cuir, 8, 209, 257, 314 (1919). 


334 THE HYDROUS OXIDES 


be expected, the sol agglomerates before it has a chance to diffuse 
into the hide substance. By adding a 30 per cent solution of 
sodium silicate to a 30 per cent solution of hydrochloric acid until 
the concentration of free acid is reduced to tenth normal, a bath 
is obtained which diffuses into the hide and deposits a protecting 
layer of hydrous silica. A fully tanned leather usually contains 
from 17 to 24 per cent of silica. One of the difficulties of the 
process is to prevent too great an adsorption of silica by the hide. | 

The most serious fault with silica-tanned leather is that it 
tears very easily after keeping for a few months.! This is prob- 
ably due to a change in the physical character of the hydrous 
silica on ageing. 

Miscellaneous Tanning, Agents.—Basic ceric chloride? can 
be used for a tan bath, giving a fairly good leather; but salts of 
bismuth have not proved satisfactory.* 

It is a remarkable fact that freshly precipitated finely divided 
sulfur is adsorbed by hide substance, giving a white leather* 
which does not swell when left for 24 hours in water and can be 
dried without losing its stability. Apostolo claims that the 
sulfur is not extracted by carbon bisulfide; but this is disputed 
by Thomas?’ who finds that sulfur is not a true tanning agent. 

Colloidally dispersed insoluble sulfides, silicates, oxides, and 
phosphates of many metals appear to act as tanning agents.°® 
Indeed, Procter’ reports that finely divided insoluble powders, 
such as ultramarine, can convert hide into leather by mere 
mechanical drumming. There is no doubt of the essential 
physical nature of the latter process. At the opposite extreme 
is the tanning action of chlorine and bromine but not iodine, where 
the change is doubtless of a purely chemical nature.’ The leather 
obtained with halogens is imputrescible and resists the action of 
cold water but not boiling water. 


1THuAv: Cuir, 9, 10, 80, 102 (1921). 

2 GARELLI: Collegium, 418 (1912). 

3 GARELLI and Apostouo: Collegiwm, 422 (1913). 

4 AposToLo: Collegium, 420 (1913). 

5 J. Ind. Eng. Chem., 18, 259 (1926). 

6 SomMMERHOFF: Collegium, 381 (1913). 

7 Bogue’s ‘‘Colloidal Behavior,’’ 2, 718 (1924). 

8 MEUNIER and SEYEWETz: Collegium, 289, 373 (1911). 


TANNING 335 


From this survey it is obvious that the term ‘‘tanning”’ has 
been applied to a wide variety of processes whereby hide fiber is 
converted into what is called “leather.’”’ It would be more 
proper to speak of ‘‘leathers,”’ for the commercial article shows 
marked variations in properties, depending on the method of 
manufacture. Procter! distinguishes the following general 
methods of tanning: 

1. By mere dehydration of the separated fibrils in such a 
way that they can be dried without adhesion. 

2. By actual changes in the chemical nature of the fibrils, which 
destroy their adhesive character. 

3. By coating the fibers with fine powders or precipitates or, 
perhaps, fatty matters, which mechanically separate them. 


1 Bogue’s “Colloidal Behavior,”’ 2, 730 (1924). 


CHAPTER XVI 
MORDANTS 


The adsorption of many dyes by wool, silk, and cotton is so 
weak that they are of value to the practical dyer only when used 
in conjunction with mordants. ‘The term mordant (from mordre, 
to bite or to corrode) was first applied by the French to metallic 
salts which were supposed to act by biting or opening a passage 
into the fibers of the cloth, giving access to the color. Thus, 
alum was believed to be effective in fixing certain dyes, owing to 
the solvent or corrosive action of sulfuric acid.t Itis now known 
that the real mordant is the hydrous oxide and not the acid 
derived from the salt. 

In general, a mordant may be defined as any substance that 
is adsorbed strongly by the cloth and, in turn, adsorbs the dye 
strongly. In dyeing a mordanted cloth, it is the mordant rather 
than the fiber which adsorbs the dye in most cases. When a 
mordant adsorbs a dye in the absence of a fiber, the product is 
called a lake. The lakes employed as pigments are usually 
prepared in contact with what are termed lake bases, such as 
barium sulfate, china clay, red lead, and lead sulfate, which 
modify the physical properties of the lakes in some desired way. 

In order to appreciate the importance of mordants in the art of 
dyeing, one needs but to recall that the first so-called direct or 
substantive dye, Congo red, was not discovered until 1884. 
Before this date it was impossible to dye cotton with acid and 
basic dyes except by the use of mordants. Moreover, substan- 
tive dyes on cotton are in general much less fast to light and 
washing than are the mordant colors. 

A typical example of a mordant dye is alizarin, the important 
coloring matter of the roots of rubia tinctorium, or madder, a plant 
of Indian origin which was cultivated largely in France and 

1 Bancrort: “Philosophy of Permanent Colors,” 1, 341 (1813); Napier: 


‘‘A Manual of Dyeing,”’ 186 (1875). 
336 


MORDANTS 337 


Holland before the synthesis of alizarin from anthracene was 
accomplished in 1868. If a piece of cotton is dipped into an 
aqueous solution of alizarin, it assumes a yellow color that is 
easily removed by washing with soap and water; but if the cloth 
is first mordanted, it is dyed a fast color: red with alumina, 
reddish brown with chrome, orange with tin, and purple or 
black with iron. By treating the fiber with the so-called sul- 
fonated oils before mordanting with alumina, there results the 
brilliant Turkey red, a color remarkable for its fastness to light 
and to the action of soap and water. The dyeing of Turkey red 
is a very ancient process having been carried out centuries ago in 
India, using milk as fatty matter and munjeet, the Indian 
madder plant. The plant itself with its earthy incrustations 
furnished enough alumina to give the color lake. The art 
spread from the East through Persia and Turkey, reaching 
France and England in the latter part of the eighteenth century. 

Wool like cotton can be dyed with madder only by the aid of 
mordants. The scarlet trousers of the French soldiers, introduced 
by Louis Philippe to encourage madder culture, and the scarlet 
uniform of the British soldier of Revolutionary war days were 
made possible by the use of the mordant alumina. 

Two classes of mordants are generally recognized: acid and 
basic or metallic. The acid mordants are the tannins, the 
fatty acids, albumin, hydrous silica, arsenic acid, and phosphoric 
acid; while the basic mordants are the hydrous oxides of the 
heavy metals. 

The most important metallic mordants are the hydrous oxides 
of chromium, aluminum, iron, tin and copper, in the order named. 
Alumina was the first mordant used, and years ago, alumina and 
stannic oxide were the most important because people were inter- 
ested in getting the bright colors which these mordants yield. 
As might be expected, the mordanting action of nearly all the 
possible oxides has been investigated. Liebermann! reports 
that the oxides of yttrium, beryllium, thorium, cerium, zirconium, 
and copper hold dyes most tenaciously; while the oxides of zinc, 
cadmium, manganese, antimony, bismuth, lead, tin, and thal- 
lium are much less satisfactory; and the oxides of iron, aluminum, 
chromium, and uranium occupy an intermediate position. Such 

1 Ber., 35, 1493 (1902). 


338 THE HYDROUS OXIDES 


a classification is not generally applicable; thus Wingraf! finds 
zirconia to be a stronger mordant for certain dyes than alumina; 
while the reverse is true in other cases. In any event, oxides of 
metals other than aluminum, chromium, iron, and tin are used 
only in special cases. For example, titania is reported to be 
a particularly good mordant to use with leather.2, The more 
important mordants will be taken up in some detail. 


ALUMINA 


If an aluminum salt which we shall represent by AIA; is dis- 
solved in water, hydrolysis takes place in accord with the fol- 
lowing equation: 


2AlA; + zH.0 @ Al.O;:2H20 + 6HA 


The reaction proceeds further to the right, the more dilute the 
solution, the weaker the acid formed, and the higher the tempera- 
ture. Whether the insoluble hydrous oxide precipitates out on 
heating or remains in colloidal solution depends on the concen- 
tration of the solution and the precipitating power of the anion. 
Crum prepared a positive sol of hydrous alumina by hydrolyzing 
the acetate and boiling off the excess acetic acid; and Neidle‘* 
obtained a sol by dialysis of a solution of aluminum chloride at 
elevated temperatures. A sol cannot be prepared by dialysis of 
the sulfate on account of the high precipitating power of sulfate 
ion. The amount of hydrous oxide formed in a given case is 
increased by removing hydrogen ion with alkali; but the range 
of hydrogen ion concentration in which the oxide precipitates 
is much wider in the case of salts with strongly adsorbed multi- 
valent ions, such as sulfate, than with salts of univalent ions. 

While the non-existence of definite basic salts of aluminum has 
not been established with certainty, it is probable that no definite 
basic compounds are formed by the hydrolysis of aluminum salts 
either alone or on the addition of alkali. Certainly, the vast 
majority of the alleged basic acetates described by Crum and of 
the basic nitrates, chlorides, sulfates, acetates, and sulfoacetates 

1 Farber-Zig., 25, 277 (1914). 

2 BaRNES: J. Soc. Dyers Colourists, 35, 59 (1919). 


3 Tiebig’s Ann. Chem., 89, 168 (1854). 
4 J. Am. Chem. Soc., 39, 71 (1917). 


MORDANTS . 0339 


formulated by Liechti and Suida! are wholly without experi- 
mental foundation. By adding alkali to aluminum sulfate, a 
phase separates below pH = 5.5 having approximately the com- 
position 5A12.03 - 3803;? but the ease with which the sulfate can 
be displaced by a wide variety of inorganic and dye cations argues 
against its being a definite compound.’ 

In view of the fact that aluminum salts hydrolyze of them- 
selves, one should expect the hydrolysis in a given case to proceed 
further in the presence of a fiber which adsorbs hydrous aluminum 
oxide. This is actually the case, as will be shown in the sub- 
sequent paragraphs. 

Mordanting of Wool.—When wool is treated with solutions of 
aluminum sulfate, Al.(SO4)3- 18H2O, less than 5 per cent on 
the wool, the bath is exhausted completely, all the alumina and 
the sulfuric acid being adsorbed.* At higher salt concentrations, 
more and more remains in the bath. Knecht? believes that both 
hydrous oxides and true basic aluminum salts are deposited 
by the mordanting process, since the spent liquors on dyeing 
well-washed wool with alizarin always possess an acid reaction. 
This evidence of basic salt formation is inconclusive, since 
adsorbed sulfuric acid would be displaced quite as readily as acid 
in definite chemical combination. Ftirstenhagen and Apple- 
yard® give data to show that the amount of sulfate taken up by 
wool remains constant when the fiber is mordanted from potash 
alum solutions containing 10 to 20 per cent of alum referred to 
the wool. According to Havrez’?’ and to von Georgievics,°® 
the amount of alumina taken up by wool from relatively dilute 
potash alum solutions is greater than the amount of sulfuric acid; 
but with increasing salt concentrations, the amount of sulfuric 


1J. Soc. Chem. Ind., 2, 537 (1883); cf. also ScotumBERGER: Bull. soc. 
chim., (3) 18, 41 (1895); BOrrincER: Liebig’s Ann. Chem., 244, 224 (1888). 

2 Miuuer: U. S. Pub. Health Repts., 38, 1995 (1923); WiLiiamson: J. 
Phys. Chem., 27, 284 (1923). 

3 See p. 379. 

4ZLiecuti and Scuwitzer: Mitt. techn. Gewerbe-Museums in Wien, Sek- 
tion fiir Fdrberet, 3, 47 (1886). 

6’ KNECHT, Rawson, and LowEntTHAL: ‘A Manual of Dyeing,’ 237 (1916). 

6 J. Soc. Dyers Colourists, 105 (1888). 

7 Chem. Zentr., 696 (1874). 

8 J. Soc. Chem. Ind., 14, 653 (1895). 


340 THE HYDROUS OXIDES 


acid taken up increases relatively to the alumina until at 24 per — 
cent alum referred to the wool, the alumina and acid are taken 
up in the same relative amounts as they occur in aluminum sul- 
fate.t Recently, Paddon,? in Bancroft’s laboratory, determined 
the amounts of both alumina and sulfuric acid removed from potash 
alum baths at different concentrations. In these experiments, — 
2-cram samples of well-washed wool were boiled for 1 hour in 
the alum solutions, after which the wool was removed and aliquot 
portions of the several baths were analyzed in the usual gravimet- 
ric manner for aluminum and sulfate. The adsorption of alumina 
and sulfuric acid is given in Tables XX VI and X XVII and shown 
graphically in Figs. 23 and 24, respectively. Both curves are 


TABLE XX VI.—ADSORPTION OF ALUMINA BY WooL 


Per cent potash Seg onan oe repre Milligram mol 
: tration Al.Os, tration Al,Os, 
alum on weight Ree ae Al,O3 adsorbed 
milligram mols | milligram mols 
of wool ; 3 per gram of wool 
per liter per liter 
5.12 0.4388 0.137 0.377 
10.25 0.881 0.395 0.607 
15.37 1.319 0.842 0.597 
20.50 1.761 1.362 0.500 
25.62 2.200 1.863 0.421 
30.75 2.645 — 2.403 0.303 


TABLE XX VII.—ApDSoORPTION OF SuLFURIC ACID By WooL 


Per cent potash Organ ae Hag act n Milligram mol 
: tration SOs, tration SOs, 
alum on weight pe he SO; adsorbed 
milligram mols | milligram mols 
of wool : : per gram of wool 
per liter per liter 

5.12 1.730 1.120 0.068 
10.25 3.460 2.550 0.113 
15.37 5.185 4.075 0.139 
20.50 6.915 5.705 0.153 
29).08 8.650 7.305 0.168 
30.75 10.375 9.055 0.165 


1Cf. THeNARD and Roarp: Ann. Chim., 74, 267 (1810). 
2 J. Phys. Chem., 26, 790 (1922). 


MORDANTS 341 


Milligram Mots Adsorbed per Gram of Wool 





Concentration of Als0,, milligram mols per liter 


Fig. 23.—Adsorption of hydrous alumina by wool. 





































Milligram Mols Adsorbed per Gram of Wool 





0 4 
Q | fs S 4 5 6 T tS) 9 
Concentration of SO3 milligram mols per liter 


Fic. 24.—Adsorption of sulfate by wool. 


342 THE HYDROUS OXIDES 


smooth and free from sudden breaks, indicating that the mor- 
danting of wool with potash alum does not lead to the formation 
of definite chemical compounds on the fiber; but that the proc- | 
ess is strictly an adsorption phenomenon, involving both 
alumina and sulfuric acid. It is probable that the acid is 
adsorbed both by the alumina and by the wool. 

The alumina curve passes through a maximum due to the pre- 
cipitation of considerable alumina on boiling the solutions of 
higher concentrations, thereby cutting down the concentration of 
alumina so far as the wool is concerned. The adsorption of SO; 
follows a continuous course, approximating saturation in the 
neighborhood of 20 per cent of alum on the weight of the wool. 
Above this concentration, the amount adsorbed is necessarily 
approximately constant; hence, this should not be construed as 
indicating the adsorption of a definite basic salt on the fiber. 

The purpose of the mordant is to have something on the fiber — 
which will adsorb and hold the coloring matter. It is, therefore, 
important to have the mordant taken up under such conditions 
that it will be held most tenaciously by the cloth, have the 
maximum transparency, and adsorb the greatest amount of dye. 
As Bancroft? points out, one would not ordinarily expect to obtain 
the mordant in such a form that it will satisfy all these require- 
ments to the maximum degree, simultaneously; but the aim 
should be to get the mordant in the form which is most generally 
useful. One objection to alum or aluminum sulfate for mordant- 
ing wool is that sulfate ion coagulates alumina too readily, thereby 
precipitating perceptible amounts of the hydrous oxide in the 
bath or superficially on the wool in a form that does not hold 
well. This is particularly true with more concentrated baths, 
as noted by Havrez? and by Paddon.* The former recommends 
a bath containing less than 10 per cent alum referred to the 
amount of wool, otherwise the mordant washes off readily and the 
wool is not dyed deeply when treated with the coloring matter. 
As one would expect, the so-called basic solutions of aluminum 
sulfate cannot be used at all with wool, since the precipitation of 


1J. Phys. Chem., 18, 399 (1914). 
2 Dinglers polytech. J., 205, 491 (1872). 
3 J. Phys. Chem., 26, 791 (1922). 


MORDANTS 343 


the hydrous oxide is altogether too rapid. Liechti and Suida! 
claim that alum does not give as good a mordant as aluminum 
sulfate. This may be due to one or more of the following causes: 
the increase in the hydrolysis of aluminum sulfate by the pres- 
ence of sodium sulfate;? the detrimental precipitating action of 
the excess sulfate in alum; and the increasing of the relative 
amount of sulfate adsorbed. Addition of sulfuric acid to alum 
causes the mordant to penetrate more thoroughly and to be fixed 
better than when the normal sulfate is used.* This is because the 
cutting down of the hydrolysis by the increased acidity is more 
pronounced than the agglomerating action of the increased con- 
centration of sulfate. 

The rapid precipitation of hydrous oxide in a loose condition on 
the surface of the cloth can be obviated by using an aluminum 
salt of a weak acid, such as aluminum oxalate, tartrate, or lactate. 
Although these salts hydrolyze more readily than sulfate, the 
resulting hydrous oxide is held in a more highly peptized state. 
Accordingly, the mordanting is deeper and more uniform from 
a solution of aluminum tartrate or oxalate; or from a solution of 
aluminum sulfate to which a mordanting assistant such as cream 
of tartar, tartaric acid, or oxalic acid is added. Some observa- 
tions of Miller* are of interest in this connection: Portions of a 
solution 0.005 M with respect to aluminum chloride and 0.0075 
M with respect to potassium oxalate were treated with gradually 
increasing amounts of alkali. No precipitate was formed until 
the pH value of the solution reached 8.8. Below this value, a 
slightly opalescent sol was formed but no floc. In striking con- 
trast to this, a 0.0025 M solution of potash alum formed a good 
floc at as low a pH value as 4.3 and up to 8.9. Obviously, the 
tendency of hydrous alumina to agglomerate under these condi- 
tions is much less in the presence of oxalate than of sulfate. It is 
probable that the behavior of tartrate is similar to that of oxalate, 
since the mordant obtained in the presence of the former is even 
more satisfactory than in the presence of the latter. 


1 J. Soc. Chem. Ind., 5, 526 (1886). 

2 LigcutTi and Supa: J. Soc. Chem. Ind., 2, 587 (1888). 

3 LiecuTi and Scuwitzer: J. Soc. Dyers Colourists, 161 (1886). 
4U. 8, Pub. Health Repts., 40, 351 (1925). 


344 THE HYDROUS OXIDES 


The beneficial influence of organic acids on the mordanting 
process has received widely diversified interpretations from time 
to time. Thus, Knecht, Rawson, and Lowenthal! claim that the 
aluminum salts of tartaric and oxalic acids possess a certain resist- 
ance to the dissociating action of wool; this is, of course, inaccu- 
rate, as the salts of the weaker acids hydrolyze more readily 
than sulfate. Beech? says that the addition of a little oxalic 
acid, cream of tartar, or tartaric acid to the mordanting bath 
helps in the decomposition of the metallic salt by the wool 
fiber; but this seems improbable, as the addition of an acid cuts 
down the hydrolysis. Herzfeld* offers no explanation of the 
phenomenon, but he recognizes clearly that the loose and uneven 
character of the mordant obtained with aluminum sulfate alone 
is due to the rapidity with which the salt decomposes; and that 
the presence of cream of tartar, oxalic acid, or lactic acid causes. 
the precipitation to take place more slowly and regularly, 
thereby giving a more satisfactory mordant. 

Mordanting of Silk.—Silk adsorbs hydrous alumina somewhat 
less strongly than wool and must, therefore, be mordanted from 
slightly more basic solutions. The solutions employed are alu- 
minum sulfate,* alum,* and the sulfate-acetate and nitrate-acetate 
mixtures. Hermann® has made observations which leave little 
room to doubt but that the real mordants are the hydrous oxides, 
at least in the case of silk. In these experiments, both raw and 
boiled-off’ silk were treated with various mordanting baths at 
30°, and the mordanted fiber was analyzed for both metallic 
oxide and acid radical. The results have been collected in Table 
XXVIII. 

Hermann looks upon mordanting as a catalytic process in 
which the fiber decomposes the mordanting salts catalytically, 
giving hydrous oxides that become fixed on the fiber and acids 


1“ A Manual of Dyeing,’ 236 (1910). 

2 “The Principles and Practice of Wool Dyeing,”’ 71 (1902). 

3 “Das Farben und Bleichen der Textilfasern,’’ 58 (1900). 

4 GANSwINDT: “Theorie und Praxis der modernen FAarberei,”’ 2, 18 (1903). 

5 KNECHT, Rawson, and LéwentHAL: ‘‘A Manual of Dyeing,”’ 238 
(1916). 

6 J. Soc. Chem. Ind., 28, 1148 (1904). 

7 Immersed in a good neutral Marseilles or olein soap solution at 90 to 
95°, in order to remove the silk gum or pericine from the fibers. 


MORDANTS 345 


TaBLE XXVIII 





Ratio of adsorbed 


Mordanting solution SEM: of oxide to adsorbed 
silk : : 
acid radical 

BEBUNIG CUIOTIOG. 2. .4.0.5... 06000). Raw LoorenOsel Cl 
MOR IeTCH OTIGG. 2.0... ce ek eee Boiled off 143 SnO2:1 Cl 
Porricsiuate (Dasic).. o6....00....-- Raw 111 Fe,03:1 SOs; 
Merme suliate (DASIC)..../....5...... Boiled off 91 Fe.,O3:1 SO3 
comic GhlOmie..c..5..-...-....:| Raw 40 Cr203:1 Cl 
Piivoiiiercnligniie..................-.| . Boiled off 44 Cr.O3:1 Cl 
PUR ChtALe. ... ss... ose eee Raw Al.O; only adsorbed 
PEN BOCLALC.. 25... ence se ses Boiled off Al.O3 only adsorbed 


that remain in the bath. While the hydrolysis of the mordanting 
salts is increased, owing to strong adsorption of the hydrous 
oxides by the fiber, the process is not catalytic, as a given amount 
of fiber can increase the decomposition of only a limited amount 
of salt, and the mordanted fiber is not in the same condition after 
the process as before. 

The mordanting of silk may be carried out satisfactorily at 
15 to 20°. At as low a temperature as 0 to 5°, the mordanting 
salts do not penetrate the fiber well, and the adsorption of the 
hydrous oxides takes place slowly and irregularly. ! 

Mordanting of Cotton.—It has been recognized for a long time 
that normal aluminum sulfate and alum cannot be used as a 
mordant for cotton.? This is because the cotton adsorbs hydrous 
alumina much less strongly than wool or silk and so does not 
decompose solutions which are distinctly acid. If the acidity 
of the alum solutions is reduced by the addition of sodium car- 
bonate, they can be used to mordant cotton. Liechti and Suida’® 
showed that the amount of alumina fixed is greater the more basic 
the mordanting solutions. Since cotton adsorbs hydrous alumina 
less strongly than wool, the mordant is fixed less strongly by 


1 Hermann: J. Soc. Chem. Ind., 22, 623 (1903); 28, 57 (1904). 

2 Cf. Bancrort: “Philosophy of Permanent Colors,” 1, 357; 2, 148, 242 
(1813). 

3 J. Soc. Chem. Ind., 2, 538 (1883); cf. Kmrrscuera and Utz: Mitt. techn. 
Gerwerbe-Museums in Wien, Sektion fiir Fdrberet, 3, 110 (1886). 


346 THE HYDROUS OXIDES 


cotton than by wool;! accordingly, we should expect the relatively 
large amounts of mordant taken up from highly basic solutions to 
rub off readily. Recently, Tingle* claimed that hydrous alumina 
is adsorbed neither from aluminum sulfate nor basic aluminum 
sulfate solutions. Hisresults with aluminum sulfate confirm those 
of everybody else, but the observations with basic aluminum 
sulfate cannot be generally true, since such solutions have been 
used in mordanting cotton without a fixing agent. 

Aluminum acetate appears to be the best mordanting bath 
for cotton. Fifty years ago, Napier* pointed out the advantages 
of acetate over sulfate: 


First, the acetic acid is not so hurtful in its action upon the vegetable 
coloring matters; second, it holds the alumina with much less force than 
sulfuric acid, and consequently yields it much more freely to the cloth; 
and third, being volatile, a great portion of the acid flies off during the 
process of drying. 


Another way of putting it is that aluminum acetate hydrolyzes 
readily, giving the hydrous oxide in the form of a finely divided 
sol which can penetrate into the fiber and be adsorbed. The use 
of aluminum formate® and aluminum lactate® in place of aluminum 
acetate has been suggested; but the principle is the same with 
all salts of weak organic acids. Cotton is not mordanted from a 
solution of ‘‘sodium aluminate,” but the latter is used to pad on 
hydrous alumina in calico printing.’ This is accomplished by 
precipitating hydrous alumina on the cloth by adding ammonium 
chloride to the aluminate bath. 

Cotton may be mordanted with alumina by first treating the 
fiber with a substance like tannin which is adsorbed strongly by 
the fiber and, in turn, adsorbs hydrous alumina strongly. This 
will be referred to again in the section on fixing agents. 


1BancrorT: J. Phys. Chem., 26, 501 (1922). 

2 J. Ind. Eng. Chem., 14, 198 (1922). 

3 KNECHT, Rawson, and LOwrentuat: “A Manual of Dyeing,’’ 233 (1916). 

4“ A Manual of Dyeing,” 121 (1875); cf. Bancrorr: ‘Philosophy of 
Permanent Colors,” 1, 365 (1813). 

> ScowaLBE: Kolloid-Z., 5, 129 (1907). 

6 BOEHRINGER and Sons: Z. Fdrben-Ind., 9, 237, 253 (1910). 

7 GanswinpT: “Theorie und Praxis der modernern Fiarberei,” 2, 212 
(1903). 


MORDANTS 347 


It is interesting to note that mercerized cotton adsorbs sub- 
stantive dyes! and takes up basic mordants? more strongly than 
ordinary cotton does. ‘This is not because the mercerized cotton 
is a definite chemical compound between cotton and sodium 
hydroxide*® as Ganswindt* assumes; but is probably due to the 
retention of sodium hydroxide in the channel of the cotton fiber 
or to a change in structure of the cotton as a result of the merceri- 
zation process. 


CHROME 


Mordanting of Wool.—Chrome is by far the most important 
mordant used with wool. More than twenty years ago Gans- 
windt® claimed that 98 per cent of all the mordanting of wool is 
done with chromic oxide; and Matthews® stated recently that 
‘‘chrome mordant is used for dyeing practically all of the alizarin, 
mordant, and acid mordant or after-chromed dyes; it is also the 
principal mordant used in conjunction with the natural logwoods.”’ 

It is interesting to note that the mordanting bath most gener- 
ally used is an acid solution of bichromate instead of a chromic 
salt.” Before the war, the readily crystallized potassium bichro- 
mate was commonly used, but the demand for a cheaper product 
led to the development of a pure crystalline form of sodium 
bichromate which has displaced the potassium salt for mordant- 
ing purposes.® 

From the bichromate solution, wool adsorbs chromic acid 
which is subsequently reduced to chromic oxide, the real mor- 
dant. Chromic acid is not held very strongly® by the fiber and 
practically all of it can be removed by washing.!®° Wool itself 


1 Marttruews: “Application of Dyestuffs,’ 165, 278 (1920). 

2 SCHAPOSCHNIKOFF and Minaserr: Z. Fdrben-Ind., 3, 165 (1904); 4, 
81 (1905). 

3 LeicHTon: J. Phys. Chem., 20, 188 (1916). 

4 “Theorie und Praxis der modernen Farberei,’”’ 2, 215 (1903). 

5 “Theorie und Praxis der modernen Farberei,’’ 2, 69 (1903). 

6 “ Application of Dyestuffs,’ 334 (1920). 

7 Knecut, Rawson, and LOwentuat: ‘‘A Manual of Dyeing,” 255 (1916). 

8 MatrueEws: ‘Application of Dyestuffs,’ 344 (1920). 

9 Liecutt and Hume : J. Soc. Chem. Ind., 12, 244 (1893). 

10 BancRoFT: J. Phys. Chem., 26, 737 (1922); cf., however, WHITELEY: 
J, Soc. Chem. Ind., 6, 131 (1887), 


348 THE HYDROUS OXIDES 


has been shown to reduce chromic acid,! but this involves more or 
less waste,” so that a reducing agent is usually added either by 
itself or in the form of a dye, such as logwood? or alizarin;* 
and under these conditions, the wool is not attacked appreci- 
ably. Chromic acid mordants wool more strongly than either 
neutral chromate or bichromate,’ so that, in practice, a suitable 
amount of acid is added to the bichromate bath. Within limits, 
increasing the acid concentration increases the amount of chromic 
acid adsorbed.’ This is less marked with sulfuric acid than 
with either hydrochloric or nitric acid, probably because sulfuric 
acid is more strongly adsorbed by wool than hydrochloric or nitric 
acid and so is more effective in cutting down the adsorption 
of chromic acid. The importance of sulfate ion is further 
indicated by the fact that a mixture of sodium chloride and 
sulfuric acid behaves like sulfuric acid and not like hydrochloric. 
The presence of sulfuric acid is more effective than an equivalent 
amount of either hydrochloric or nitric acid in causing the oxidation 
of wool by chromic acid. Since the oxidizing power of chromic 
acid is greater the higher the concentration of acid, and since 
sulfuric acid is adsorbed by wool more strongly than hydrochloric 
or nitric acid, Bancroft’ attributes the greater effect of the former 
to higher acid concentration at the surface of the wool. 

A bichromate bath acidified with sulfuric acid is objectionable, 
not only because the reduction of chromic acid takes place at 
the expense of the wool, but because some chromic oxide remains 
in the mordant and oxidizes such colors as alizarin blue, aliz- 
arin yellow, etc., producing weak shades that may be undesir- 
able.2 As a matter of fact, the more customary thing is to use 
an organic acid or acid salt such as cream of tartar, tartaric 
acid, oxalic acid, formic acid,® and lactic acid.‘° As these 


1Lrecuti and Humme.: J. Soc. Chem. Ind., 12, 244 (1893). 

2 DurFEE: Am. Dyestuff Rep., 9, No. 10, Tech. Sec. 20-23 (1921). 

3 Marruews: “Application of Dyestuffs,’ 477 (1920). 

4 Liecuti and Humme.: J. Soc. Chem. Ind., 12, 244, 246 (1893). 

5 HumMMEL and Garpner: J. Soc. Chem. Ind., 14, 452 (1895). 

6 BancrortT: J. Phys. Chem., 26, 743 (1922). 

7 J. Phys. Chem., 26, 744 (1922). 

8 BeEcH: “The Principles and Practice of Wool Dyeing,’’ 116 (1902). 

9 Kappr: Z. Fdrben-Ind., 4, 159 (1905); WuirraKeEr: “ Dyeing with Coal 
Tar Dyestuffs,’ 50 (1919). 

10 KNECHT, Rawson, and LOwWENTHAL: ‘‘A Manual of Dyeing,” 173, 256 
(1916). 


MORDANTS 349 


so-called assistants are oxidized by chromic acid, it is probable 
that there is little, if any, oxidation of the wool in their presence. 
Moreover, they bring about a uniform deposit of the mordant 
in a form highly satisfactory for receiving the dye.! 

Solutions of chromium salts undergo hydrolysis to a greater 
or lesser degree, depending on the basicity of the solutions, the 
concentration, and the temperature. Wool adsorbs hydrous 
chromic oxide from such solutions in the same manner as hydrous 
aluminum oxide is adsorbed from aluminum salts. If chrome 
alum is used, the fiber takes up sulfuric acid as well as the 
hydrous oxide. Liechti and Hummel claim that a part of the 
acid is taken up as a basic salt having the formula 3Cr.03-2SQ3. 
Their data do not justify this conclusion, but the absence of a 
basic salt has not been proved. Williamson’ obtained a gel of 
approximately constant composition by precipitating chrome 
alum below a certain pH value which was not determined. The 
amorphous mass was assigned the formula 7Cr.03:4SO3. It will 
be recalled that Williamson* and Miller® obtained a gel of approxi- 
mately constant composition, 5Al,03:38O03, by adding alkali to 
alum below pH = 5.5. For reasons already given,® I do not 
consider the alumina sulfuric acid gel to be a definite basic salt 
and the same applies to the chrome sulfuric acid gel. However, 
at least one definite crystalline basic sulfate of the formula 
[Cr(OH)2(H.2O)4]2°SO4 has been prepared;’ so the formation of 
a basic salt on the fiber must be regarded as a possibility. 

It is claimed that chrome alum cannot be used for a mordant- 
ing bath, because the mordant is not adsorbed evenly and the 
subsequent dyeing is uneven. Since a good mordant can be 
obtained with aluminum alum, it would appear that the difficulty 
with chrome alum could be corrected by suitable adjustment of 
the temperature or of other conditions of mordanting. The 
addition of cream of tartar, oxalic acid, or tartaric acid to the 


1KneEcHT, Rawson, and LOwentHau: “A Manual of Dyeing,” 256 
(1916); Begcu: “The Principles and Practice of Wool Dyeing,” 117 (1902). 

2 Liecuti and ScuwirzeEr: J. Soc. Chem. Ind., 4, 586 (1885). 

3 J. Phys. Chem., 27, 384 (1923). 

4 J. Phys. Chem., 27, 280 (1923). 

5 U.S. Pub. Health Repts., 38, 1995 (1923). 

6 Cf., pages 339, 379. 

7 Werner: Ber., 41, 3447 (1909). 


300 THE HYDROUS OXIDES 


alum bath gives a satisfactory mordant as does chromium oxalate! 
or chromium tartrate but not chromium acetate or chromium 
fluoride.? 

Liechti and Hummel* observed increased mordanting with 
increasing concentration of chrome alum, just as would be 
expected. They also claimed to get an increased amount of chro- 
mium taken up by increasing the sulfuric acid content of the 
alum bath; but this is improbable, if not impossible, unless the 
heating is conducted in such a manner that a precipitate forms in 
the bath and is padded on the fibers. The reported increase in 
adsorption with increasing sulfuric acid content is contradicted 
by the further observation of Liechti and Hummel that the 
bath is exhausted less completely the greater the concentration 
of sulfuric acid. 

‘Wool is mordanted very slightly from solutions of chromic 
chloride or chromic nitrate,4 probably because the degree of 
hydrolysis is less and the peptizing action of these solutions for 
hydrous chromic oxide is too great to yield the mordant to the 
fiber. If this be true, the addition of a suitable amount of soda 
to chromic chloride HURUEOS should give a satisfactory mordant- 
ing bath. 

Mordanting of Silk.—Silk adsorbs chromic oxide less strongly 
than wool.’ In practice, it is mordanted from a bath of chrome 
alum® or chromic chloride but not from bichromate.’ To pre- 
' serve the luster of silk, Whittaker® recommends mordanting the 
silk overnight in a cold bath of chromic chloride, followed by 
treating with sodium silicate, which fixes the mordant on the fiber. 

Mordanting of Cotton.—Cotton adsorbs hydrous chromic oxide 
very much less readily than either wool or silk, as evidenced by 
the observation that no mordanting whatsoever results on heating 
cotton with a 10 per cent chrome alum solution. Apparently, no 
completely satisfactory chrome mordant for dyeing cotton, 

1 Tag iant: Color Trade J., 11, 158 (1922); Textile Colorist, 44, 650 (1922). 
— 2 Lrecuti and HuMMEL: i sun Chem. Ind., 13, 356 (LSE 

3 J. Soc. Chem. Ind., 18, 222, 356 (1894). 

4 Liecuti and Hosa J. foe Chem. Ind., 18, 224 (1894). 

6’ GANSWINDT: ‘Theorie und Praxis der modernen Farberei,”’ 2, 19 (1903). 

6 Liecuti and HummgE.: J. Soc. Chem. Ind., 13, 223 (1894). 


7 KNECHT, Rawson, and LOwentnuat: ‘‘A Manual of Dyeing,’ 258 (1916). 
8 “Dyeing with Coal Tar Dyestuffs,’ 50 (1919). 


MORDANTS 351 


especially cotton yarns, has been found.! The most satisfactory 
bath is the colloidal solution of hydrous chromic oxide in alkali, 
the so-called alkali chromate bath.? This cannot be used for 
yarns on account of the caustic action on the hands of the work- 
men; nor can it be used on oiled material, since the oil would 
be stripped from the fiber. A bath of chromic acetate is fairly 
successful, as the acetic acid may be removed by heating. 


Iron MorpbDANTS 


Mordanting of Wool.—At one time, ferrous sulfate was widely 
used for mordanting wool; but it has been largely replaced by 
chrome mordants. The iron mordant is still of importance in 
dyeing logwood blacks, since the latter on chrome mordant are 
likely to turn green on exposure to light. Moreover, it is claimed 
that cloth mordanted with copperas posesses a ‘‘kinder’”’ and 
softer handle than cloth mordanted with chrome. In general, 
iron mordants tend to “‘sadden”’ or darken the shade of most 
dyes, and they are, therefore, used chiefly for dark colors, espe- 
cially browns and blacks. 

A copperas black may be obtained either by mordanting 
before dyeing or mordanting after dyeing. The latter process, 
which is usually. employed, consists essentially in boiling the wool 
in a decoction of dyewoods for a time and then adding copperas 
directly to the bath. When the fiber is mordanted before dyeing, 
it is necessary to add comparatively large quantities of tartar or 
oxalic acid to prevent unequal precipitation of the oxide of iron 
on the fiber. Before placing the mordanted cloth in the dye 
bath, better results are obtained by allowing it to le for several 
hours in the air, whereby hydrous ferrous oxide is oxidized more 
or less completely to the ferric state. From this, it would appear 
either that hydrous ferric oxide is a better adsorbent than fer- 
rous oxide or that the oxidation of the mordant following dyeing 
may have a detrimental effect on the final product. 

Mordanting of Silk.—Iron salts are quite extensively used in 
mordanting silk for dyeing black, especially with logwood. 
Alumina and tin mordants are of minor importance and chrome 

1 KNECHT, RAwSsoN, and LowenrHan: “A Manual of Dyeing,” 252 


(1916). 
2 Korncuuin: Dinglers polytech. J., 254, 132 (1884). 


302 THE HYDROUS OXIDES 


is seldom used as a mordant for logwood; nor is logwood used to 
produce any color on silk other than black. For the dyeing of 
silk, mordants are applied in sufficient amount not only to take 
up the dye but to add appreciably to the weight of the silk. 
Raw silks adsorb the hydrous oxide fairly strongly; but it is 
customary to impregnate the fiber with tannin before putting it 
in the iron bath which is usually ferrous acetate. By repeated 
treatment in the tannin and salt baths, the weight of the silk 
fiber may be increased as much as 400 per cent. If a ferric salt 
such as basic ferric sulfate is employed, the fiber is first mordanted 
with the hydrous oxide which is subsequently ‘‘fixed” in a tannin 
bath. 

While raw silk adsorbs and holds the hydrous iron oxides 
fairly strongly, boiled-off silk possesses but a slight adsorption 
capacity for the mordant. The latter is, therefore, dipped in 
the iron liquor and subsequently put into a boiling soap solution 
containing olein soap and a little soda, which precipitates 
hydrous ferric oxide on the fiber in an aged condition. This 
operation may be repeated several times according to the amount 
of weighting desired. 

Mordanting of Cotton.—Cotton shows a much weaker adsorp- 
tion for hydrous ferric oxide than either wool or raw silk. It is, 
therefore, mordanted by a process similar to that employed with 
boiled-off silk, namely by saturating in a solution of basic ferric 
sulfate followed by treating with lime water or soda solution, 
which precipitates the hydrous oxide in the cloth. If ferrous 
sulfate is employed, the fiber is first mordanted with tannin, 
which adsorbs the hydrous oxide strongly; and any sulfate 
adsorbed is subsequently removed by washing with lime water. 
After mordanting, the adsorbed hydrous oxide is allowed to 
oxidize in the air before placing in the dye bath. 


Tin Morpants! 


Mordanting of Wool.—Although wool is seldom mordanted 
with tin mordant, when this is done, the bath consists of stan- 
nous chloride in conjunction with oxalic acid or tartaric acid. 
Considerably more acid is said to be taken up from stannic salt 
baths than from stannous salt baths, which accounts for the use 

1 Cf. p. 210, 


MORDANTS 353 


of the latter in practice. As in the case of alumina mordanting, 
tin salts require the presence of an organic acid to prevent rapid 
and uneven deposition of the hydrous oxide on the fiber. Stan- 
nous tartrate and stannic tartrate alone are said to be unsatis- 
factory; but it is possible that the addition of an excess of either 
tartaric acid or oxalic would made a good mordanting bath if 
there were any point in avoiding the use of chloride. The 
hydrous oxide of tin is sometimes “‘fixed”’ with alum. 

Mordanting and Weighting of Silk.—The most important use 
of tin salts in the dyeing industry is in the mordanting and weight- 
ing of silk. For this purpose, stannic chloride is the salt gener- 
ally employed. ‘The cloth is first steeped in a solution of this 
salt, and after rinsing, is put into a bath of sodium phosphate 
and subsequently into one of sodium silicate. In order to give 
the silk the desired weight,” the process must be repeated several 
times. . 

If the silk is weighted excessively by the tin-phosphate-silicate 
process, serious faults may develop in the goods. Thus, heavily 
weighted silk frequently becomes quite tender when exposed 
even for a short time to direct sunlight.* Moreover, reddish- 
colored tender spots often appear in pieces, after storing. Gne- 
hm, Roth, and Thomann’ first attributed the formation of these 
tender spots to the action of perspiration; but this cannot be 
true, as unused goods frequently show the damaged spots. Sis- 
ley® pointed out that the only constituent of perspiration which 
has an injurious effect is the salt; and Meister® showed that the 
deterioration of the silk is due to active chlorine produced by the 
catalytic action of copper which is always present in small quan- 
tities as a result of careless handling during spinning and weaving. 
As a preventive, Knecht’ suggest padding the goods in a very 
weak solution of ammonium thiocyanate; but this is not infal- 


1HEERMANN: J. Soc. Dyers Colourists, 1903-1906; NruHaAvs: Knecht, 
Rawson, and Léwenthal’s “‘A Manual of Dyeing,” 279 (1916). 

2 GNEHM and BarnzicER: J. Soc. Dyers Colourists, 40 (1897). 

3 KNECHT, Rawson, and LOwentHAL: “A Manual of Dyeing,” 279 
(1916). 

4 J. Soc. Dyers Colourists, 256 (1902). 

5 J. Soc. Dyers Colourists, 276 (1902). 

6 J. Soc. Dyers Colourists, 192 (1905). 


354 THE HYDROUS OXIDES 


lible. The use of thiourea and its salts has been patented for the 
same purpose. ! 

Since silk adsorbs hydrous stannic oxide, leaving most of the 
hydrochloric acid in the bath, the latter becomes strongly acid 
by continued use. To keep the bath in good condition, stannic 
chloride must be replaced and the excess hydrochloric acid neu- 
tralized with ammonia from time to time. After the ammonium 
chloride content of the liquor becomes too high for satisfactory 
mordanting, a fresh bath must be employed. 

Mordanting of Cotton.—Stannic salts are sometimes used to 
mordant cotton; but on account of the usual weak adsorption of 
cotton for the hydrous oxides, the fiber must first be mordanted 
with tannin. When sodium stannate is used, the cloth is first 
impregnated with a solution of the salt and is then passed through 
a very dilute solution of sulfuric acid or of aluminum sulfate. 
Hydrous stannic oxide or, if an aluminum salt is employed, a 
mixture of the hydrous oxides of tin and aluminum are precipi- 
tated and constitute the mordant. 


TANNIN 


Having considered the most important basic mordants, it 
seems advisable to point out the essential characteristics of a 
typical acid mordant. The class of substances known as the 
tannins, to which tannic acid belongs, is seldom employed in 
mordanting wool but finds its chief use in mordanting cotton 
and linen, in “‘fixing”’ the hydrous oxide mordants on cotton, and 
in weighting silk with hydrous ferric oxide, as noted in an earlier 
paragraph. 

Both wool and cotton adsorb tannin from its colloidal solution 
in water, the amounts taken up varying continuously with the 
concentration of the sol, as shown by the curves in Fig. 25 con- 
structed from the data of Pelet-Jolivet? on the adsorption by 
wool and of Sanin® on the adsorption by cotton. 

The adsorption of tannin by wool is not very marked, especially 
at ordinary temperatures; but it increases with the temperature; 


1J. Soc. Dyers Colourists, 51 (1907). 
2“TDie Theorie des Farbeprozesses,’’ 79 (1910). 
3 Kolloid-Z., 10, 82 (1912). 


MORDANTS 395 


on the other hand, the adsorption by cotton apparently decreases 
with increasing temperature of the bath.!_ If mixed cotton goods 
containing wool are mordanted at ordinary temperature, the cot- 
ton only is mordanted to any appreciable extent. 

Since tannin is an acid mordant, one might expect the adsorp- 
tion to be reduced in alkaline solution and increased in acid 
solution. As a matter of fact, the adsorption of tannin is cut 
down almost to zero in the presence of alkali; and acetic acid 
increases the adsorption? which, however, passes through a 


Concentration of Tannin, grams per !00 cc. in Mordanting Cotton 





Adsorption, grams Tannin per gram Fiber 


0.05 010 015 020 025 030 035 040 OAS 
Concentration of Tannin, grams per 100 ce. 


Fig. 25.—Adsorption of tannin by wool and cotton. 


maximum at high concentration.* Sulfuric acid, on the other 
hand, cuts down the adsorption, and hydrochloric acid has little 
effect. This behavior with different acids is probably due to 
the difference in the adsorption of the acids by cotton. We 
know, for example, that sulfuric acid is adsorbed by cotton more 
strongly than hydrochloric,* which would account for the 
adsorption of tannin being cut down more by the former than by 
the latter. Different salts added to the bath all seem to increase 


1Knecut and Kersuaw: J. Soc. Dyers Colourists, 40 (1892); Gan- 
SwINDT: ‘Theorie und Praxis der modernen Fiarberei,’’ 2, 216 (1903). 

2 Knecut, Rawson, and Léwentuau: “A Manual of Dyeing,” 1, 188 
(1916). 

3 DreaPeER: “The Chemistry and Physics of Dyeing,’ 161 (1906). 

4LeicHtTon: J. Phys. Chem., 20, 188 (1916). 


306 THE HYDROUS OXIDES 


the adsorption of tannin, possibly because they decrease the 
stability of the sol. 

Although tannin is adsorbed quite strongly by cotton, it must 
be ‘‘fixed’’? on the fiber before the dyeing process. The best 
fixing agents are antimony salts; but salts of tin, aluminum, 
and iron are used in special cases. 


Frxinc AGENTS 


Whenever a mordant is not fixed sufficiently strongly by a fiber, 
it is necessary to add a so-called fixing agent to bring about the 
desired results. For example, sodium phosphate is used for 
fixing alumina and tin; sodium arsenate, soap, and tannin for 
iron; sodium silicate and tannin for chrome and tin; salts of 
antimony, tin, and aluminum for tannin; etc. In other words, 
arsenates, silicates, phosphates, fatty acid salts, and tannin are 
used as fixing agents for the basic or metallic mordants while the 
latter are used for fixing the acid mordants, tannin, and the fatty 
acid compounds. ‘The question arises as to whether the fixing 
process consists in the formation of definite chemical compounds, 
such as antimony tannate, iron arsenate, tin phosphate, etc., 
as is generally assumed, or whether the fixed mordants are mix- 
tures of indefinite composition. The latter view seems much 
the more reasonable in the light of the evidence. For example, 
it is known that precipitated hydrous ferric oxide is peptized as a 
positive sol on washing and that tannin is peptized by water 
as a negative sol. If the two are brought in contact, there is 
mutual adsorption and each keeps the other from being peptized; 
in other words, there is a mutual “‘fixing.’”” The so-called iron 
tannates are not definite compounds. 

The case of the action between antimony salts and tannin has 
been studied by Sanin! who believes there are at least three 
definite antimony tannates. These cannot be obtained pure; 
but at first, Sanin preferred to regard different products as 
mixtures of two or more antimony tannates rather than as sub- 
stances of continuously varying compostion. Later,? he con- 
cluded that adsorption does occur when tannin and potassium 


1%, Farben-Ind., 9, 2, 17, 49 (1910). 
2 Sanin: Kolloid-Z., 18, 305 (1918). 


MORDANTS 357 


antimony tartrate are mixed; but that tannates are formed 
during the technical procedure used in mordanting with tannin. 
While no one can question the possibility of forming a true anti- 
mony tannate under special conditions, it is altogether improbable 
that the varied conditions in technical practice are such as to 
yield a definite salt. 

Wislicenus and Mutte! studied the action of tannin on fibrous 
alumina. The amount taken up was found to increase rapidly 
at first, with increasing concentration of the sol and then to 
reach an approximately constant value. This constant value is 
the limiting value of adsorption? for the particular alumina and 
does not indicate the formation of aluminum tannate. Had a 
different alumina been used, the saturation value would have 
been found at a different point. Von Schréder* showed that 
the taking up of tannin from solution in alcohol and from the 
aqueous sol is a typical adsorption phenomenon and no tannate is 
formed. 

It is a moot question whether the fixing of iron oxide or alumina 
by oil mordants is due to the formation of salts of fatty acids. 
Knecht‘ says: 


The amount of iron which is taken up by the fiber depends less on 
the strength of the mordanting liquor than on the amount of oil that 
has already been fixed in the material; the oil attracts the oxide of iron 
with great energy, so that it is not readily stripped from the fiber even 
by comparatively concentrated sulfuric acid or hydrochloric acid. 


This behavior is more nearly what one would expect if the 
ferric oxide were adsorbed by the oil than if a ferric salt of a 
fatty acid were formed; but there is no proof either way.® 

Turning to the fixing of metallic mordants by phosphates, 
silicates, etc., we know that compounds are not formed, in many 
cases. Thus, hydrous aluminum oxide adsorbs arsenic acid;® 


1 Kolloid-Z., 2, 2d Supplement, XVIII (1908). 

2Scumipt: Z. physik. Chem., 74, 699 (1910). 

3 Kolloidchem. Beihefte, 1, 1 (1909). 

4Knecut, Rawson, and Lowentuat: ‘“‘A Manual of Dyeing,’ 2, 597 
(1916). 

5 Cf. BAncroFT: J. Phys. Chem., 19, 50 (1915). 

6 LOCKEMANN and Paucke: Koilloid-Z., 8, 273 (1911). 


358 THE HYDROUS OXIDES 


hydrous ferric oxide adsorbs arsenious acid! and arsenic acid; 
hydrous stannic oxide adsorbs phosphoric acid;? and beryllium 
oxide adsorbs arsenious acid.* In none of these cases is a definite 
compound formed; and it is probable that many more of the 
alleged phosphates, stannates, and silicates of the heavy metals 
are not obtained under ordinary conditions. However, this 
does not preclude the formation of definite compounds under 
special conditions. Thus, crystalline aluminum orthophosphate?* 
results on treating a concentrated solution of sodium aluminate 
with an excess of phosphoric acid and heating in a sealed tube 
at 250° for several hours. 

It was claimed for a long time, that alizarin would dye alumina 
only in the presence of lime salts, the color on the mordanted 
cloth being attributed to a calcium aluminum alizarate.> This is 
now known to be true only in case the hydrous oxide contains 
sulfate. Hydrous alumina prepared from aluminum acetate 
takes up alizarin in the absence of calcium salts.6 The purpose 
of the calcium is not to fix the alumina to the fiber or the dye 
to the mordant, but to remove sulfate, which cuts down the 
adsorption of the alizarin.’ 


Cotor LAKES 


In the dyeing of mordanted cloth, the color is taken up chiefly 
by the mordant giving a color lake on the fiber. For a long 
time, all lakes were believed to be definite compounds between 
the metallic oxide and the dye. This view was questioned by 
Biltz and Utescher,* who investigated the behavior of alizarin with 
hydrous chromic oxide and hydrous ferric oxide. With the 
former, the amount of dye taken up increases continuously with 
increasing concentration of solution, giving no indication what- 
soever of the formation of a chromium alizarate. On the other 


1 Britz: Ber., 37, 3138, 3151 (1904). 

2 MECKLENBURG: Z. physik. Chem., 74, 207 (1912). 

3 BLEYER and Miitumr: Arch. Pharm., 251, 304 (1913). 
4 Dr ScHuuTEen: Compt. rend., 98, 1853 (1884). 

5 LigcutTi and Surpa: J. Soc. Chem. Ind., 5, 525 (1886). 
6 Davison: J. Phys. Chem., 17, 737 (1913). 

7 BancrortT: J. Phys. Chem., 18, 10 (1914). 

8 Ber., 38, 4143 (1905). 


MORDANTS 359 


hand, with hydrous ferric oxide, there is a rather marked increase 
in the amount of dye taken up with relatively small change in the 
concentration of the bath, leading Biltz to conclude that iron and 
alizarin combine in a definite ratio of 1 molecule of iron to 3 of 
alizarin.t But since the amount of alizarin taken up by the 
iron oxide is so far in excess of that necessary to form alizarate, 
one is confronted by the necessity of assuming either that ali- 
zarate adsorbs the excess dye or that the whole phenomenon is 
a case of adsorption of dye by the hydrous oxide. 

If sodium alizarate reacts with hydrous ferric oxide to form a 
ferric alizarate, it must follow that sodium hydroxide will be 
liberated by the reaction; while if the sodium alizarate is adsorbed 
by the hydrous oxide, there should be no accumulation of alkali 
in the solution. Bull and Adams? investigated the phenomenon 
quantitatively. It was necessary, first of all, to determine the 
adsorption of sodium hydroxide by hydrous ferric oxide, since 
ferric alizarate might form and still leave little alkali behind in 
case the latter were sufficiently strongly adsorbed by the excess 
oxide. Observations were then made on the amount of alkali 
remaining in solution on shaking the hydrous oxide with sodium 
alizarate prepared by dissolving resublimed alizarin in a very 
slight excess of the theoretical quantity of alkali. The relative 
quantities of dye and oxide were so chosen that practically com- 
plete adsorption of the dye occurred even at the highest concen- 
trations. The results are given in Tables X XIX and XXX and 
shown graphically in Fig. 26. | 


TABLE X XIX.—ApsorpPTION oF NAOH sBy Hyprovus FE,.O3 


N/10 NaOH at start, N/10 NaOH at end, N/10 NaOH adsorbed, 


cubic centimeters cubic centimeters cubic centimeters 
1.20 0.20 1.00 
2.40 0.56 1.84 
3.60 1.07 2800 
4.80 Ba 4.) 
6.00 2.13 Bereyé 


1 Cf. also LizcutTsE and Surpa: J. Soc. Chem. Ind., 5, 523 (1886). 
2 J. Phys. Chem., 25, 660 (1921). 


360 THE HYDROUS OXIDES 
TABLE XX X.—ADSORPTION OF SopIuM ALIZARATE BY Hyprowus FER.O; 


N/10 NaOH 





Sodium alizarate : N/10 NaOH N/10 NaOH 
: equivalent to ; 
solution, in bath, adsorbed, 
: : the alizarate, : : . : 
cubic centimeters ; : cubie centimeters | cubic centimeters 
cubie centimeters 
5.00 ya 0.15 1,20 
10.00 PA EY 0.25 225 
15.00 3.00 0.30 3.45 
20.00 5.00 ou 4.65 
25.00 6.25 0.35 5.90 


If ferric alizarate were formed, alkali would be liberated as 
given in column 2 of Table XXX. Much smaller quantities of 
this are found, and there is also much less sodium hydroxide 
- present in the baths than would be found if the calculated amount 





Cc.of 0.1 N NaOH Adsorbed 
w 


eae 
it 
7 a 


1.5 1.8 “al 24. 
Cc. of 0.1 N NaOH in Solution 
Fic. 26.—Adsorption by hydrous ferric oxide. 





of alkali were formed and subsequently adsorbed by the ferric 
oxide as shown in Table X XIX. The small quantities of alkali 
recorded in column 3 of Table XXX are due to hydrolysis of the 
adsorbed sodium alizarate, producing in solution the amounts of 


MORDANTS O61 


alkali shown, while the insoluble alizarin remains on the fiber. — 
The first two values are lower than the hydrolysis value because 
the amount of hydrolysis will be determined to some extent by 
the intensity of adsorption. Further experiments were carried 
out, which eliminated the possibility that the adsorption of alkali 
was increased by the presence of sodium alizarate. In the 
light of these observations and the continuous curve obtained 
by Biltz, it seems altogether probable that the iron-alizarin lake 
is an adsorption complex and not ferric alizarate. 

Liechti! claims that a definite aluminum alizarate is obtained 
when hydrous aluminum oxide and sodium or ammonium aliza- 
rate are brought together. This claim was found to be altogether 
without foundation by Williamson? who investigated the matter 
in much the same way as Bull and Adams did the iron-alizarin 
lakes. 

Marker and Gordon? studied the influence of hydrogen ion 
concentration on the amount of the basic dyes, crystal violet, 
and methylene blue, and the acid dyes, orange II, and metanyl 
yellow, taken up by hydrous ferric oxide, alumina, and silica. 
The different hydrogen ion concentrations were obtained by 
the addition of sulfuric acid or sodium hydroxide. Some data 
are given in Tables XX XI and XXXII. In all cases, it will be 
seen that the amount of dye taken up increases with increasing 
pH for basic dyes and decreases with increasing pH for acid dyes. 
Throughout the range investigated, the pH-adsorption curves 
appear continuous for the adsorption of crystal violet and metanyl 
yellow by alumina and of crystal violet for ferric oxide. On 
the other hand, the amount of methylene blue taken up increases 
greatly between pH = 11 and 12, and the same is true for orange 
II between pH = 3.2 and 2.3; so that these curves are drawn 
with a sharp break at approximately pH = 11 and pH = 3.2, 
respectively. Since crystalline salts of a number of metals, 
including iron and aluminum, can be obtained by the action of 
the sulfonic acid, orange II, on the respective oxides, it is con- 
cluded that the breaks in the pH adsorption curves show the 
lakes investigated to be definite compounds. While certain ones 

1J. Soc. Chem. Ind., 4, 587 (1885); 5, 523 (1886). 


2 J. Phys. Chem., 28, 891 (1924). 
8 J. Ind. Eng. Chem., 16, 1186 (1924); 15, 818 (1928). 


062 THE HYDROUS OXIDES 


TaBLE XXXI.—ApDsoRPTION oF Dyes By Hyprous FERRIC OxIDE 
(Adsorption in milligrams dye per gram of gel) 
































Basic dyes Acid dyes 
Methylene blue Crystal violet | Orange II | Metanyl yellow 
Adsorp- Adsorp- Adsorp- Adsorp- 
Re tion no tion pH tion ne tion 
1.96 27.6 2.06 | 23.2 2.30 | 429.0 1.92.40 5361-0 
2.33 29.0 2.94 | 33.0 3.20 75.0 2.30 | 340.0 
5.95 30.0 5.02 | 42.3 5:27 70.0 3.38 | 255.0 
9.85 32 9.01 50.6 10.14 52.0 7.46 | 211.0 
tT) .12 33.8 10.95 | 56.1 11.02 50.0 | 11.60 80.7 
12.00 | 131.0 


TABLE XX XII.—ApDSORPTION OF DyrEs BY Hyprous ALUMINA 
(Adsorption in milligrams dye per gram of gel) 





























Basic dyes Acid dyes 
Methylene blue Crystal violet | Orange IT | Metanyl yellow 
Adsorp- Adsorp- Adsorp- Adsorp- 
pe tion i tion pe tion pH tion 
1.96 65.6 1.50 3.0 2.30 | 452.0 1.92 | 703.0 
2.23 66.1 5.44 8.0 3.20 186.0 2.30 | 460.0 
5.95 67.5 9.18 45.0 5.27 179.0 7.46 | 276.0 
9.85 77.0 10.70 | 282.0 10.14 162.0 9.67 | 226.0 
11.12 82.7 Tia 4s. 0 11.02 136.0 11.60 115.0 
12.00°| 27920 . 


of the lakes may be definite salts, I cannot see how this can be 
deduced from the adsorption data or the curves constructed 
therefrom. Consider the orange II lake with either alumina or 
ferric oxide: If the amount of dye taken up by a definite amount 
of oxide were independent of the concentration of the dye bath 
at a certain pH value, then the assumption that the lake is a 
definite salt might be justified. But the above data show only 
that a change in pH value from 5.2 to 3.2 causes a much smaller 


MORDANTS 363 


increase in adsorption than a change in pH value from 3.2 to 2.3. 
If the equilibrium 


Fe20 


lak 3 
eee ee LK UX 





(where X is the acid dye anion) exists as assumed by Marker and 
Gordon, there is nothing to indicate why the velocity to the right 
should proceed regularly with decreasing pH to a certain point, 
and then jump abruptly. Had the adsorption been determined 
at two or three points between pH = 3.2 and 2.3, it is probable 
that the adsorption curve would prove to be a continuous one 
with a sharp bend, instead of a broken one that has no apparent 
significance. It is possible that the marked increase in adsorp- 
tion above pH = 3.2 is due to the concentration of hydrogen ion 
being sufficiently great to cause some peptization of the oxide 
on boiling for an hour. If the experiments were repeated with a 
freshly formed gel, I should expect the amount adsorbed for a 
given pH value to show considerable variation from the values 
obtained by Marker and Gordon. | 

The effect of the concentration of dye on the amount taken up 
at various constant pH values should be investigated. Marker 
and Gordon determined the amounts of dyes left over on treating 
different concentrations of orange II with an excess of hydrous 
oxide at constant pH. They fail to give the important thing, 
namely, the constant pH value of the solution; but one is pretty 
safe in assuming that it was low, as all the dye anion was taken up 
except a constant small amount, the equilibrium concentration. 

For methylene blue to form a salt, hydrous ferric oxide must 
function as an acid. Until someone shows that a very weak base 
like that of methylene blue will react with ferric oxide to give a 
stable salt even under special conditions, there seems no ground 
for assuming that the iron-methylene blue lake as ordinarily 
obtained is ever a definite compound. Pelet-Jolivet! showed 
that methylene blue is adsorbed by silica, the amount taken up 
depending upon the previous history of the hydrous oxide. 

That it is unsafe to assume a lake to be a true compound simply 
because the constituents in question can form a definite salt under 
special conditions is further emphasized by the work of Gilbert? 


1“Die Theorie des Farbeprozesses,”’ 71, 205 (1910). 
2 J. Phys. Chem., 18, 586 (1914). 


364 THE HYDROUS OXIDES 


on the copper lakes of eosin. Gilbert prepared a definite crystal- 
line copper eosinate; but found it to be a different substance from 
the precipitate obtained by the interaction of copper sulfate and 
sodium eosinate. Although the precipitated lake has a fairly 
constant composition, it always contains an excess of copper 
when an excess of copper salt is employed. By shaking hydrous 
copper oxide with varying concentrations of an ether solution of 
eosin, a typical adsorption isotherm is obtained, showing no 
evidence of compound formation. The maximum amount of 
eosin adsorbed under these conditions is only about one-tenth of 
that necessary to form copper eosinate. Starting with colloidal 
hydrous copper oxide and colloidal eosin acid, lakes were obtained 
varying in composition between 2 molecules of copper to 1 of 
eosin and 2 molecules of eosin to 1 of copper. All the lakes 
behave like the one in which copper and eosin are in equivalent 
quantities, and all can be carried into colloidal solution. In 
the presence of ether, small amounts of certain salts decompose 
the lakes. This is because the adsorption of the anions of the 
salts by hydrous copper oxide is sufficiently great to displace the 
adsorbed eosin. The order of displacing power of the anions is 
the usual order of adsorption: SO4”” > Br’ > Cl’ > NO3’. 

The taking up of crystal ponceau by wool mordanted with 
alum was shown by Pelet-Jolivet! to be a clear case of adsorption 
when the process is carried out at room temperature; but at 90°, 
the amount taken up is practically independent of the concen- 
tration of the dye bath, when the latter contains more than 700 
milligrams per liter, the lowest concentration employed. It is 
probable that, in this instance, a definite aluminum salt of crystal 
ponceau is formed. Pelet-Jolivet prepared such a salt in crys- 
talline form. 

Bayliss? found that hydrous alumina adsorbs Congo red acid 
from its deep-blue colloidal solution in water. If this adsorption 
complex is washed, suspended in water, and heated, the color 
changes from blue to red. Since Congo red salts are red, Bayliss 
attributed this change in color to the formation of an aluminum 
salt. The experiments were extended to the precipitates obtained 
by mixing the negative sol of Congo red acid with the positive sols 


1 “Die Theorie des Farbeprozesses,”’ 213 (1910). 
* Proc. Roy. Soc., 84B, 881 (1911). 


MORDANTS 365 


of the hydrous oxides of aluminum, zirconium, and thorium. 
The blue adsorption complex became red on heating in every 
case, provided the hydrous oxide sols were dialyzed until practi- 
cally free from acid. A small amount of acid is sufficient to pre- 
vent the color change. Assuming that the color change is due 
to the formation of a Congo red salt, Bancroft! fails to see why a 
trace of acid should prevent the change, provided there is an 
excess of hydrous oxide with which the Congo red can react. 
Blucher and Farnau? attempt to get around this difficulty by 
assuming that hydrous alumina adsorbs and stabilizes the free 
red Congo acid which is instable in aqueous suspension. This 
raises the question why a trace of free mineral acid should prevent 
the adsorption and alleged stabilization of the red Congo acid. 
The whole phenomenon should be reinvestigated quantitatively, 
paying attention to the relative concentrations of both the posi- 
tive and negative sols and their respective pH values. In this 
connection it may be mentioned that Schaposchnikoff and Bogo- 
jawlenski® have isolated the metastable red Congo acid by allow- 
ing the pyridine salt to effloresce. 

Considering the acid mordant, tannin, for a moment, we find 
Dreaper* stating that magenta and tannic acid form a definite 
salt; but no proof is offered for the statement, and the admission 
is made that 100 parts of the alleged salt will take up at least 160 
parts of tannin when the latter is present in excess. Sanin® 
likewise states that basic dyes form definite salts with tannin 
when the dyes are in excess; but when tannin is in excess, the 
latter is adsorbed. In no case is any proof.of compound forma- 
tion presented except the fact that a formula for the alleged 
product can be written. 

In conclusion, one seems justified in saying that mordants 
function by adsorbing dyes in indefinite proportions depending 
on the conditions. In certain cases, definite salts may be formed, 
but these constitute the exceptions to the general rule. 


1 J, Phys. Chem., 19, 57 (1915). 

2 J. Phys. Chem., 18, 634 (1914). 

8 J. Russ. Phys.-Chem. Soc., 44, 1813 (1918). 

4“The Chemistry and Physics of Dyeing,” 244 (1906). 
®Sanin;: Z, Farben-Ind., 10, 97 (1911). 


CHAPTER XVII 
WATER PURIFICATION 


All natural waters are contaminated to a greater or lesser 
degree by the materials with which they come in contact. Thus, 
waters from regions of old rocks like granite are relatively low in 
mineral content; while waters from regions of limestone are hard. 
Surface waters flowing through districts containing readily pep- 
tizable material like clay are more or less turbid, and those from 
swampy regions are highly colored. 

The purification of water on a large scale is carried out with 
one or more of the following objects in view: first, to render the 
supply safe and suitable for drinking; second, to reduce the 
amounts of mineral ingredients which are injurious to boilers; 
and third, to remove substances injurious to the machinery or 
the manufactured product in industrial processes. The col- 
loidal matter in surface waters consists of finely divided particles 
of clay, sand, organic coloring matter, and bacteria. Such 
material can usually be removed by agglomeration and filtering 
under suitable conditions. Undesirable dissolved substances 
such as the bicarbonates and sulfates of calcium and magnesium 
can be eliminated only by resorting to chemical precipitation. 
Many of the largest artificial water purification plants are 
operated solely to provide potable water without special atten- 
tion to its use for industrial purposes. In other instances, the 
water is not only rendered potable but is softened at the same 
time. A notable example of the latter is the purification plant 
at New Orleans where hard, colored, turbid, sewage-polluted 
water from the Mississippi River is rendered suitable for indus- 
trial as well as domestic consumption. 

The most important requirement in the purification of a 
municipal water supply is the elimination of bacteria, especially 
those causing disease, and the removal of turbidity; but a per- 
fectly acceptable drinking water is free from objectionable odor, 

366 | 


WATER PURIFICATION 367 


taste, and color. Small amounts of the mineral constituents 
commonly found in water are not objectionable, as a rule, but 
certain ones are highly undesirable. Thus, the presence of as 
little as 2 p.p.m. of iron renders the water unpalatable to some 
people and causes trouble by discoloring washbowls and tubs, 
and by producing rust stains on cloth. It is needless to say 
that drinking water must contain no more than a trace of salts 
of barium, copper, zinc, and lead, because of their poisonous 
character. Fortunately, the occurrence of harmful amounts of 
the latter salts in the ordinary water supply is quite rare, 


FILTRATION 


Surface water is rendered potable by filtration, sometimes 
accompanied by disinfection with ozone, chlorine, or hypochlorite 
which destroy disease-producing organisms, and by the addition 
of an algicide such as copper sulfate to kill organisms responsible 
for objectionable tastes or odors. Chemical treatment alone is 
not a substitute for purification by filtration, since it does not 
remove colloidal matter which causes turbidity and color, or 
dissolved organic matter which produces swampy tastes and 
odors. 

Two general types of filters are employed in purifying municipal 
water supplies: slow sand filters and mechanical filters. 

Slow Sand Filtration.—In slow sand filtration the water is 
caused to pass through a suitable layer of sand which removes 
the undesirable suspended matter. The method was inaugurated 
by Simpson in England in 1829 and is frequently referred to as 
the English system. The filter is a very large water basin con- 
taining filtering material 1.5 to 2 meters in thickness. The upper 
layer consists of fine sand approximately 1 meter in thickness 
supported on coarser sand, and this in turn, on a layer of graded 
gravel, the coarest material at the bottom. Drains are installed 
below the gravel to carry off the filtered water. 

The process of slow sand filtrations is about as follows: The 
raw water containing suspended material, together with colloidal 
clay, bacteria, microscopic plants, etc., is run into asedimentation 
basin where part of the impurities settle out under the influence 
of gravity. The removal of microorganisms by the filter is not 


368 THE HYDROUS OXIDES 


very efficient until the surface layer of the sand becomes coated 
with a slimy protoplasmic deposit called the ‘‘schmutzdecke.”’ 
This protoplasmic filtering laver consists essentially of myriads 
of living forms—diatoms, fungi, blue and green algz, protozoa, 
and bacteria—together with silt, mud, and other colloidal 
matter. Although the greater part of the impurities are retained 
in the surface layer, thick filter beds have been found to be more 
efficient than thinner ones, indicating that each particle of sand 
contributes to the purification. Obviously, the rate at which the 
filtration is carried out has an important bearing on the effi- 
ciency of the process. In practice, from two to four million gal- 
lons of water per acre per day are rendered substantially free 
from suspended matter, including bacteria. When the proto- 
plasmic film has become clogged so that the rate of filtration is 
unduly retarded, the water is allowed to subside below the sur- 
face, about 144 inch of sand is. scraped off, and the filtering 
resumed. The sand is used over again after washing free from 
impurities. 

The slow sand filter is suited to purification of waters contain- 
ing relatively small amounts of color, suspended matter, and 
animal pollution. This type of filter has been in use in Europe 
for years and has proved most efficient; on the other hand, but 
few American waters can be treated successfully and economically 
by this process. In some places where the slow sand filter has 
been adopted and has not proved entirely satisfactory, the 
normal biological action of the filter is supplemented by the use 
of coagulants, such as aluminum sulfate. The trivalent alum- 
inum ion causes agglomeration of the negatively charged col- 
loidal particles and hydrous aluminum oxide, which subsequently 
settles out, carries down with it a large portion of the impurities. 
Thus, at Washington, the Potomac River water is treated with 
aluminum sulfate before it enters the Georgetown reservoir, which 
acts as a sedimentation basin. After partial clarification by 
sedimentation, the water is conducted to the filter bed, where the 
undesirable impurities are further reduced. Clark? suggests 
loading the sand with coagulant as a means of supplementing 


1 PinrKe: Z. Hyg., 7, 115, 170 (1889). 
2J. Am. Water Works Assoc., 36, 385 (1922); Public Works, 68, 197 
(1922), 


WATER PURIFICATION 369 


the action of the slow sand filter. From 75 to 225 tons of alum- 
inum sulfate per acre have been used in practice. 

Aeration, preferably by spraying, before filtration brings about 
the precipitation of dissolved iron as hydrous ferric oxide. 
Objectionable odors and tastes are likewise best removed by 
aeration, either before or after filtration. 

Mechanical Filtration—The method of rapid sand filtration 
was developed in America and is, therefore, referred to as the 
American system. ‘The process is characterized by the artificial 
formation of a surface filtering layer consisting essentially of 
hydrous aluminum or ferric oxide, by the method of cleaning the 
filters, and by the rapid rate of filtration which may be as much 
as fifty times that in slow filters. The method is eminently 
suited to the treatment of turbid and highly colored waters, and is 
commonly used where softening as well as filtration is necessary. 
The process is substantially as follows: The raw water passes 
through a meter which measures the volume of water passing 
and at the same time regulates the rate of addition of coagulant 
to the flow of water. If the water is to be softened, it is next 
passed to a set of weirs where it is divided, one small fraction 
receiving the charge of lime and a second the requisite amount 
of soda ash. ‘These portions are subsequently mixed with the 
main body of water, which is then allowed to stand until the body 
of the precipitate settles out. The water is next conducted to 
the filters, which consist of concrete or wooden basins having a 
filtering area of 50 to 120 square meters. The top layer of a 
filter is of fine sand about 75 centimeters thick followed by a 30- 
centimeter layer of graded gravel, which rests on perforated 
brass strainers connected with the drain system. The small 
residual amount of suspended hydrous oxide quickly forms a 
filtering layer on the sand, which entrains the remaining impuri- 
ties. Usually after 8 to 12 hours’ operation, the filters become 
clogged and must be washed. This is accomplished by forcing 
clean water up through the strainers, thus dislodging the impuri- 
ties which pass over the top of the filters with the wash water. 
Both gravity and pressure filters are in use, but the principle is 
the same in each. 

The precipitate of hydrous alumina or ferric oxide adsorbs and 
entangles practically all suspended matter including bacteria; 


370 THE HYDROUS OXIDES 


but where the raw water has a very high bacterial count, it may 
be necessary to sterilize the water with chlorine, hypochlorite, 
or ozone as an added precaution against transmitting’such diseases 
as typhoid fever and Asiatic cholera. 

Flinn, Weston, and Bogert! summarize the applicability of 
slow sand and rapid sand filters, as follows: 


For a water having a turbidity? less than 30 p.p.m.? or a color? less 
than 20 p.p.m., slow filters without coagulation give excellent results. 
For waters having a turbidity of more than 50 p.p.m. or a color of more 
than 30 p.p.m., mechanical filters give unquestionably better results. 
They not only produce an equally safe water but one of far better 
appearance. Between these extremes is a region where either the 
mechanical filter or the slow filter with coagulants may be used equally 
well. Under ordinary conditions, the latter is far more expensive than 
the former, . 


Tue ACTION OF COAGULANTS 


When a coagulant such as aluminum sulfate is added to pol- 
luted water, several colloidal processes take place, most important 
of which are the neutralization of colloidal particles by adsorption 
of ions, followed by agglomeration; and the adsorptive action of 
the highly gelatinous aluminum oxide. The strongly adsorbed 
aluminum ion has a marked precipitating action on colloidal clay, 
bacteria, and coloring matter. Observations of the effect of 
multivalent cations on the sedimentation of clay and on the 
agglutination of bacteria date back to the pioneer work of 
Bodlander® and Bechhold,® respectively, More recently, Saville’ 


1 “Waterworks Handbook,” 734 (1918). 

2 The standard of turbidity is a water which contains 100 p.p.m. of pre- 
cipitated fuller’s earth in such a state of fineness that a bright platinum 
wire 1 millimeter in diameter can just be seen when the center of the wire 
is 100 millimeters below the surface of the water and the eye of the observer 
is 1.2 meters above the wire. The turbidity of this standard water is 100. 

3 Parts per million. 

* The standard color solution, having a color of 500, contains 1.246 grams 
K.PtCle, 1 gram CoCl,-6H:O and 100 cubic centimeters of concentrated 
HCl in 1 liter. 

5 Jahrber. Mineral., 2, 147 (1893). 

6° Z. physik. Chem., 48, 385 (1904). 

7J. New Engl. Weve Works Assoc., 31, 78 (1917). 


WATER PURIFICATION d7v1 


showed that the color taken up by water originating in swamps 
or peaty soils is due almost exclusively to negatively charged 
colloidal particles which are coagulated by the cations of the 
coagulant. Miller! confirmed this result and demonstrated fur- 
ther that the decolorizing action on the so-called humic acid 
colors is due, for the most part, to the agglomerating action of 
aluminum ion, hydrous alumina alone playing an unimportant 
role in the process. The precipitating action of hydrogen ion 
on the negatively charged impurities is much less than that of 
aluminum ion of the same concentration.2. The high precipitat- 
ing power of sulfate ion neutralizes any positively charged 
colloids that may be present in the water; but the most important 
function of a multivalent negative ion is to prevent the formation 
of a positive sol of hydrous aluminum oxide. 

The highly gelatinous hydrous alumina which precipitates 
under suitable conditions adsorbs and entangles the finely divided 
impurities, leaving the water relatively clear and uncontaminated 
as it settles out. Because of the outstanding role of the hydrous 
_ oxide in the purification process, it is important to know what 
constitutes the most satisfactory floc and how the desired product 
may be obtained. Some waters contain sufficient iron to produce 
a good floc when lime or soda ash is added; others have normally 
sufficient alkali to precipitate hydrous alumina on the addition 
of aluminum sulfate; still others require the addition of both 
sulfate and alkali. If ferrous sulfate is added, lime must always 
be used to bring about satisfactory precipitation. In practice, 
the coagulant most used is commercial aluminum sulfate, com- 
monly called filter alum or alum. A great deal of empirical 
information regarding the use of coagulants has been collected; 
but the principles underlying their proper use have received but 
little attention until recent years. In this connection, the 
important work of Clark, Thierault, and Miller in the Hygienic 
Laboratory of the United States Public Health Service deserves 
special mention. | 

Formation of Alumina Floc.—It is well known that hydrous 
aluminum oxide does not separate from an aluminum sulfate 
solution when the final solution is either too acid or too alkaline. 


1U.8. Pub. Health Repts., 40, 1472 (1925). 
2 Of. BanEeRJI: Indian J. Med. Research, 11, 695 (1924). 


372 THE HYDROUS OXIDES 


In other words, there is a comparatively narrow range of hydro- 
gen ion concentration in which a precipitate forms and the range 
of complete precipitation is still narrower. ‘The ideal conditions 
should result in the rapid and complete formation of a floc that 
settles readily. 


Rina | 
ieee 
ei ACE 















) 


Time. minutes 





40. «35 45 as we 65 70 
pl : 


Fia. 27.—Relation between time required for the appearance of floc in solu- 
tions buffered at various pH values when the total salt concentration is con- 
stant and the alum concentration is varied: 1 = 400 p.p.m.; 2 = 300 p.p.m.; 
3 = 200 p.p.m.; 4 = 100 p.p.m. 


In any precipitation process, the highest rate of precipitation 
will result under otherwise constant conditions, when the highest 
concentration of separable material above the equilibrium con- 
centration is attained. In the present instance, the degree of 
supersaturation of water with hydrous aluminum oxide can be 
varied either by increasing the total amount of aluminum sulfate 
added at a given final pH value or by varying the pH value with 


WATER PURIFICATION 373 


a constant amount of aluminum sulfate. The maximum degree 
of supersaturation and rate of precipitation of hydrous alumina 
under varying conditions was first studied by Thierault and 
Clark. The procedure was as follows: A definite volume of a 
solution of known composition was treated with varying amounts 
of aluminum sulfate in dilute solution. After rapid mixing, the 
liquid was poured into a 100-cubic-centimeter cylinder, and 
the time necessary for the first appearance of a floc was noted. 
The visibility of the floc was increased by slight agitation of the 
cylinder. In Fig. 27 is given the time required ‘for the first 
appearance of a floc in solutions buffered at various pH values, 
when the total salt concentration is constant and the alum is 
varied. The concentration of aluminum sulfate in parts per 
million is 400, 300, 200, and 100 for curves 1, 2, 3, and 4, respec- 
tively. The optimum pH value for producing a floc in mini- 
mum time increases slightly as the concentration of alum 
decreases. In curves 1, 2, 3, and 4, it is at pH = 4.95, 5.10, 
5.25, and 5.40, respectively. With less than 100 p.p.m. of 
aluminum sulfate, the optimum pH value is close to 5.5, which is 
significant since the amount used in practice is ordinarily con- 
siderably less than 100 p.p.m. Moreover, as the concentration 
of aluminum sulfate decreases, the width of the curves decreases, 
the optimum zone, using 100 p.p.m., being less than one pH unit. 

Attention is called to the fact that the floc which appears first 
in a series of experiments is always the best so far as the floccu- 
lent appearance is concerned. Also, the floc formed in mini- 
mum time is most abundant and settles most rapidly. 

The actual time required for the appearance of a floc depends 
on the size of the vessel. This difference is quite marked, a 
precipitate forming within a minute in a large vessel and often 
requiring hours to become visible in a small one. Apparently 
this ‘“‘volume effect’’ is only the effect of the volume-surface 
ratio upon circulation, since mechanical circulation decreases 
the time required for the appearance of a floc.?, Gentle agitation 
was found to influence only the time of flocculation and not the 
amount of precipitate or the optimum pH value for maximum 
rate. 


1U. 8. Public Health Repts., 38, 181 (1923). 
2 Cf. Hoover: J. Am. Water Works Assoc., 11, 582 (1924). 


374 THE HYDROUS OXIDES 


Since the two branches of the curve relating pH value to floccu- 
lation time, tend to become parallel, no floc is likely to appear for 
a very long time in laboratory vessels if the pH value is beyond 
the asymptote to either branch. Moreover, since the two 
branches come closer together the more dilute the solutions, 
the region in which a floc appears may be quite narrow for 
extremely dilute solutions. Thierault and Clark found that if 
a floc does not appear within a few hours with slight occasional 
agitation, it will not appear within a greatly reduced time with 
mechanical agitation. This indicates the necessity for rigid 
control of the final pH values under large volume conditions, in 
order to secure floc formation in a reasonable time from highly 
dilute aluminum sulfate solutions. 

The above method of determining the optimum conditions for 
a satisfactory floc was found to be applicable to natural waters 
containing carbonates. A slight but definite buffer action is 
obtained in the region of pH = 5.5 with aluminum sulfate and 
a hydroxide. At as low a pH value as this, the carbon dioxide 
of the air is not so effective in disturbing the equilibrium of dilute 
solutions as it is in such solutions nearer neutrality. Accordingly, 
it is possible to obtain mixtures of aluminum sulfate and calcium 
hydroxide having definite pH values in the range 4.6 to 6.0. 
Using M/500 calcium hydroxide without the use of supplemen- 
tary buffers and varying the amount of aluminum sulfate, the 
optimum pH value is between 5 and 6; with M/500 sodium 
hydroxide and varying amounts of aluminum sulfate, the best 
floc is obtained at pH = 5.2; and with constant amount of 
aluminum sulfate and varying amounts of alkali, the optimum 
value is about pH = 5.8. These observations would indicate 
that a hydrogen ion concentration between pH = 5 and 6 is 
best suited for the coagulation of aluminum sulfate even in 
natural waters. This proves to be approximately true in many 
cases but not in all. A minimum in residual alum in filter 
effluents under commercial conditions was found by Buswell 
and Edwards! at pH = 6; by Baylis? between pH = 5.5 and 
7.0; and by Hatfield? between pH = 5.8 and 7.5. The latter 

1 Chem. Met. Eng., 26, 826 (1922). 


2 J. Am. Water Works Assoc., 10, 365 (1928). 
3 J. Am. Water Works Assoc., 11, 554 (1924). 


WATER PURIFICATION 375 


values are for Lake St. Clair water in which the maximum rate of 
flocculation is between pH = 6.1 and 6.8. Dallyn and Dela- 
porte’ found the optimum condition for coagulation to be pH = 
5.5 and 6.5 for soft colored, and for clear Great Lakes water, 
respectively; and Mum? obtained the most favorable results with 
Triliwong River water at pH = 5.5 to 6. On the other hand, 
Hatfield* obtained most satisfactory results at Highland Park, 
Mich., when the pH value of the treated water was between 7.2 
and 7.3; and similar conditions exist in other places.4- Obviously, 
therefore, the hydrogen ion concentration most favorable for 
coagulation varies with the nature of the water. In this con- 
nection, the importance of research being carried out under the 
conditions which obtain in actual practice cannot be emphasized 
too strongly. Different waters present their individual problems 
which very frequently cannot be solved simply by referring to 
data obtained with pure chemicals. Some of the factors which 
influence the optimum hydrogen ion concentration for obtaining 
a good floc will be considered in order. 

The observations recorded in the preceding paragraphs were 
all carried out with aluminum sulfate. The range of hydrogen 
ion concentration over which flocculation occurs might be expected 
to vary with the nature of the anion. Thus, it will be recalled 
that colloidal hydrous alumina is formed by dialysis of a solution 
of aluminum chloride to which ammonium hydroxide is added 
short of precipitation; aluminum sulfate cannot be substituted 
for aluminum chloride on account of the strong precipitating 
action of sulfate ion. This is illustrated further by some 
observations of Miller® on the effect of various anions on the — 
zone of precipitation of hydrous alumina. For example, 0.005 
M solutions of aluminum chloride and aluminum sulfate were 
treated with varying amounts of alkali, and after precipitation, 
the hydrogen ion concentration of the supernatant solution and 
the amount of aluminum in the precipitate were determined. 


1 Contract Record, 37, 343 (1923); cf. CatteTT: J. Am. Water Works Assoc., 
11, 887 (1924). | 

2 Mededeel-Burgerlyken Geneeskund. Nederland-Indie, Part 1, 27 (1925). 

3 J. Ind. Eng. Chem., 14, 1038 (1922). 

4 BaANERJI: Indian J. Med. Research, 11, 695 (1924). 

5 U.S. Pub. Health Repts., 40, 351 (1925). 


376 THE HYDROUS OXIDES 


Referring to Fig. 28, it will be seen that with aluminum sulfate, 
practically complete precipitation occurs between pH values of 
about 5.3 and 8.7;! while with aluminum chloride, flocculation 
occurs only in the narrow range between pH = 7.8 and 8.6. It 
should be emphasized that these ranges represent zones of 
flocculation and not of insolubility. The insoluble hydrous oxide 
formed in the presence of chloride remains in colloidal solution 
throughout the lower pH values on account of the strong stabiliz- 
ing action of hydrogen ion and the weak precipitating power of 






=) 
S 
Oo 
as 





Mofs Al Precipitated 
= 
Cae | 
GS 


3. 4 5 (GUN ti =?ihcnt 
pH 
Fria. 28.—Zones of hydrogen ion concentration in which flocculation occurs for 
alum and aluminum chloride. 


chloride ion. Similar observations were made by Miller with 
more dilute solutions approaching those used in the actual opera- 
tion of water purification. It is quite evident, therefore, that the 
nature and precipitating power of the anions present in solution 
are equal in importance to that of the hydrogen ion concentration 
in controlling the formation of a suitable precipitate of hydrous 
alumina, From the results of observations with various anions, 
Miller reports that sulfate yields a floc best suited to successful 
water clarification. The range of concentration over which 


1 Cf. GREENFIELD and BuswEuu: J. Am. Chem. Soc., 44, 1485 (1922). 


WATER PURIFICATION Old 


precipitation occurs is broad and the floc is of good quality, rapid 
settling, and shows least tendency to become colloidally dispersed. 

Since the maximum rate of flocculation of pure aluminum sul- 
fate in dilute alkali is at pH = 5.5, Thierault and Clark suggest 
that this value may represent the isoelectric point! of hydrous 
aluminum oxide. Hatfield? likewise refers to his values of 
pH = 6.1 to 6.3 as indicating the ‘‘apparent”’ isoelectric point 
of the hydrous oxide. Asa matter of fact, the zone of maximum 
rate of flocculation is of no significance whatsoever as a direct 
experimental method of determining the isoelectric point of 
hydrous alumina, since the zone can be varied at will by varying 
the anions present in solution. Miller confirmed Thierault 
and Clark’s value of pH = 5.5 as the approximate point of 
maximum precipitation for aluminum sulfate; but the maximum 
is at pH = 8 for aluminum chloride. 

In addition to the effect of the negative ion content of natural 
waters, the presence of colloidal inorganic or organic matter, 
which may function as a protective colloid, will cause variation 
in the zone of hydrogen ion concentration in which flocculation 
occurs. ‘Thus, colloidal silica’ prevents the formation of hydrous 
alumina under certain conditions; and sewage-polluted water 
requires more coagulant than an unpolluted water having the 
same turbidity and color. 

Instead of adding aluminum sulfate directly to water, Coxe‘ 
suggests adding a colloidal solution® prepared by mixing 40 grams 
of crystalline aluminum sulfate in 80 cubic centimeters of water 
with 10 grams of sodium carbonate in 40 cubic centimeters of 
water. This sol is precipitated simply by dilution, and it is 
claimed to have certain advantages as a clarifying agent over 
aluminum sulfate and alkali added separately. Thus, clarifica- 
tion can be brought about without softening, if desired; and the 
carbon dioxide content of the water is not increased as a result 
of decomposition of aluminum sulfate in the water treated. 
The clearest advantage would appear to be in the very short time 


1 Cf. Heyrovsky: J. Chem. Soc., 117, 11, 695, 1013 a 
2 J. Am. Water Works Assoc., 11, 554 (1924), 
3Smitu: J. Am. Chem. Soc., 42, 160 (1920). 
4Chem. Met. Eng., 29, 279 (1923). 
5 Cf. SpencER: Chem. Age, 32, 31 (1924). 


3718 THE HYDROUS OXIDES 


necessary for the formation of the floc, as compared with the 
usual process. On account of the low concentration of aluminum 
ion, the sol would probably be unsuited for treating waters con- 
taining large amounts of negatively charged coloring matter. 

In view of the importance of aluminum ion in the coagulation 
and removal of coloring matter, it would appear advantageous 
to treat highly colored waters at a low pH value where aluminum > 
ions exist in solution as such, followed by increasing the pH 
value in order to precipitate all the aluminum. This is exactly 
what Norcom! does with Cape Fear River water at Wilmington, 
N. C. The desired result is accomplished by connecting two 
sedimentation basins in series, treating the water with alum at 
a low pH value in the first basin, and increasing the pH value 
in the second basin by the addition of alkali. 

Finally, it may be said that successful water purification by 
alum depends on the presence of a certain minimum quantity 
of aluminum ion; the presence of an anion of high precipitating 
power, such as sulfate; and the proper adjustment of the hydrogen 
ion concentration.’ 

Composition of the Alumina Floc.—When aluminum sulfate is 
added to water, an equilibrium is set up that may be represented 
by the following equation: 


Al2(SO4)3 -+ xH.O — Al,O3 7 xH2O + 6H’ aie 35804” 


The addition of alkali displaces this reaction to the right, com- 
plete precipitation of the aluminum resulting when approxi- 
mately 2.5 equivalents of hydroxyl to 1 of aluminum are added. 
Miller? determined the composition of the precipitate formed at 
various final pH values: Liter quantities of 0.005 M solution of 
potassium alum were added to varying quantities of sodium 
hydroxide. The pH values of the resulting solutions were 
determined, and the precipitates were analyzed for their alu- 
minum and sulfate content, after thorough washing. The results 
are given in Fig. 29. From the lowest pH value at which a pre- 
cipitate forms up to pH = 5.5, the composition of the precipitate 


1J. Am. Water Works Assoc., 11, 97 (1924); cf. Mituer: U. S. Pub. 
Health Repts., 40, 1479 (1925). 

2 Miuter: U.S. Pub. Health Repts., 40, 365 (1925). 

3 U.S. Pub. Health Repts., 38, 1995 (19238). 


WATER PURIFICATION 379 


remains constant and may be represented approximately by the 
formula 5AI,03-3S03.!. Above pH = 5.5, which corresponds 
to 2.4 equivalents of alkali to 1 of aluminum, the sulfate content 
of the precipitate decreases gradually, becoming zero at pH = 9 
when exactly 3 equivalents of alkali to 1 of aluminum have been 
added. 
The constancy of composition of the precipitate thrown down 
below pH = 5.5 suggests that it may be a basic salt. This view 





Mol Ratio Al to SO4, 


Fig. 29.—Composition of the precipitate from alum at varying hydrogen ion 
concentration. 


is rendered improbable by the ease with which the sulfate is 
displaced by washing the hydrous oxide containing sulfate, 
with solutions of negative ions of equal or greater valence.” 
Dyes containing two or more acid groups such as the di-, tri-, 
and tetrapotassium sulfonates of indigo likewise displace sulfate. 
From a study of the reciprocal displacement of oxalate and sul- 
fate ions, Miller? suggests that the negative ions are in solid 


1 Wiuiamson: J. Phys. Chem., 27, 284 (1923); cf., however, Hopkins: 
J. Am. Water Works Assoc., 12, 425 (1924). 

2 CHarRiovu: Compt. rend., 176, 679, 1890 (1923). 

3 U, S. Pub. Health Repts., 39, 1502 (1924). 


380 THE HY DROUS OXIDES 


solution in the hydrous oxide. This question was considered in 
an earlier chapter! and the conclusion was reached that the carry- 
ing down of ions by hydrous alumina is an adsorption phenom- 
enon rather than a case of solid solution in which the ions form 
an integral part of the space lattice of the microcrystals. A 
possible explanation of the constancy of adsorption of sulfate 
ion below pH = 5.5 is that the adsorption of hydrogen ion by 
the hydrous oxide reaches the saturation value at approximately 
this point. If such be the case, the amount of sulfate ion which 
must be adsorbed to neutralize the adsorbed hydrogen ion will 
be constant below pH = 5.5. Above this value, the adsorption 
of hydrogen ion falls off, and there is a corresponding gradual 
decrease in the adsorption of sulfate until it becomes zero at 
pH = 9 and above. 

It will be recalled that Thierault and Clark obtained the best 
and most rapid flocculation in very dilute aluminum sulfate 
solutions near pH = 5.5, where the precipitation of aluminum 
first approaches completion on the addition of alkali and where 
the greatest proportion of sulfate is found in the precipitate. 
Miller likewise found the precipitate to be more dense, more 
rapid settling, more opaque, less gelatinous in appearance, and 
less voluminous in the more acid portion of the flocculation range 
than at the higher pH values. It would appear that the best floc 
for commercial water clarification should be sufficiently gelati- 
nous to adsorb and entangle all impurities but sufficiently dense 
to settle rapidly. 

The Ferric Oxide Floc.—Ferrous sulfate in conjunction with 
lime is a very good coagulant for turbid alkaline waters such as 
those of the Missouri and Ohio basins. If ferrous sulfate is 
added to such water, the action with calcium bicarbonate may be 
represented as follows: 


FeSO. + Ca(HCOs3)2 = Fe(HCOs)2 + CaSO. 


Ferrous bicarbonate oxidizes and precipitates too slowly for 
practical use, and so lime must be added which precipitates 
hydrous ferrous oxide thus: 


Fe(HCO3)2. + Ca(OH). + xH2,0 = FeO: xH20 + Ca(HCQs)e 


el agtte BA E 


WATER PURIFICATION 381 


By adding sufficient lime, the calcium is also precipitated as 
carbonate. Hydrous ferrous oxide is slightly soluble; but it is 
oxidized by the oxygen dissolved in the water, giving hydrous 
ferric oxide, the coagulant desired. This oxide may be obtained 
directly from ferric sulfate but the latter salt is much more 
expensive than the ferrous salt. 

Hydrous ferric oxide forms a denser coagulum than hydrous 
alumina, and ferrous sulfate is considerably cheaper than alu- 
minum sulfate. On the other hand, ferrous sulfate must always 
be used in conjunction with lime and the mixture is not suitable 
for soft waters, because any surplus lime gives the water a caustic 
alkalinity. Moreover, hydrous ferric oxide does not remove 
coloring matter so well as hydrous alumina and so is not suitable 
for clarification of waters which are high in color or which are 
alternately turbid and colored. ‘The failure of ferrous sulfate to 
remove coloring is probably due to the relatively low precipitating 
power of ferrous ion as compared with aluminum ion and the 
rapidity with which the former is removed from solution when 
used in conjunction with lime. 

Miller! extended the study of aluminum compounds to the 
corresponding compounds of iron and found the same factors 
which determine the optimum conditions for forming an alumina 
floc to apply equally to ferric oxide floc. The floc from ferric 
alum is precipitated almost completely near pH = 3, approxi- 
mately 2.5 pH units below the zone of maximum precipitation of 
hydrous alumina from sulfate solution. Like aluminum chloride, 
ferric chloride forms a sol at lower pH values, complete floccula- 
tion occurring at approximately pH = 5.0; but unlike alumina, 
hydrous ferric oxide is insoluble at higher pH values. The zone 
of precipitation of hydrous ferric oxide is, therefore, much wider 
than that of hydrous alumina, a circumstance which may be of 
distinct advantage under certain conditions. 


1U. 8. Pub. Health Repts., 40, 1413 (1925). 


CHAPTER XVIII 
CEMENT 


The term cement, as ordinarily used at the present time, refers 
to mortars which possess the property of hardening in water as 
well as in air. Reference has already been made to hydraulic 
cements in which magnesia or zinc oxide is the most important 
constituent, so that this chapter will deal for the most part, 
with what is known as Portland cement. 


PORTLAND CEMENT 


The need for a cementing material to bind sand and small 
stones together was recognized from the time man started to 
build. In some of their constructions, the Assyrians and Baby- 
lonians are known to have used moistened clay, which was 
probably the first cementing material ever used for building 
purposes. Such a binder is not sufficiently durable or hard for 
building massive constructions, and the next development appears 
to have been the discovery by the Egyptians of the cement now 
known as plaster of Paris, which was mixed with sand to make the 
mortar used in the construction of the Pyramids. The discovery 
that the application of heat to certain rock minerals, such as 
gypsum, would give a cementing substance was later employed 
by the Greeks in making lime from limestone or marble. The 
Greeks prepared some very satisfactory mortars by mixing lime 
with sand or with sand and volcanic earths known as pozzolana. 
The development of pozzolana mortars was brought to a high 
state of perfection by the Romans, as evidenced by many of 
their imposing structures which still exist. The so-called Roman 
or pozzolana cements were similar in many respects to the modern 
Portland cement. 

The art of cement making declined with the fall of Rome and 
was not revived until 1756 when John Smeaton discovered that a 
clayey limestone found in Cornwall would give an hydraulic 
lime, when burned, This product was mixed with pozzolana 

382 


CEMENT | 383 


to prepare the mortar used in constructing the Eddystone light- 
house. Because of the scarcity of pozzolana, which is found only 
in a few volcanic regions, subsequent investigations were carried 
out in an attempt to produce an artificial Roman cement. The 
invention of a satisfactory process is attributed to Joseph 
Aspdin of Leeds, who took out a patent in 1824 for making a 
cement by heating an intimate mixture of limestone and clay at 
the temperature ordinarily used in burning lime. To this 
product, Aspdin gave the name Portland cement, since its 
color, after hardening, was similar to that of Portland stone, a 
famous English building stone. Aspdin’s original cement was 
not what is now known as Portland cement, as the temperature 
of burning was not high enough; but a year later, in 1825, the 
importance of heating the mass to incipient fusion was recognized. 
From this beginning, a century ago, there has developed the 
modern Portland cement industry, the importance of which in 
our present-day civilization is difficult to overestimate. 
Formation.—Portland cement is produced by heating a mixture 
of compounds containing suitable amounts of aluminum, calcium, 
and silicon together with small amounts of iron and magnesium. 
In the early stages of the development of the industry, the method 
of procedure employed in making a satisfactory cement was 
determined by the method of trial and error. Now, it is known 
that certain definite compounds impart the desired properties to 
cement and that a uniform product made up of these compounds 
_ results only when the raw material containing calcium, aluminum, 
and silicon in rather definite proportions is ground to a fine powder 
and the intimate mixture heated to a minimum temperature. 
Typical raw materials employed in cement manufacture are 
limestone and clay, both of which are found in large deposits of 
uniform composition. In some places, there exist deposits of 
clayey limestone, called cement rock, containing all three of the 
essential constituents; but as a rule, either limestone or clay must 
be added to get the desired composition for good Portland cement. 
The alumina and silica are sometimes derived from blast-furnace 
slag and the calcium oxide from sea shells. From whatever 
source the material is obtained, the separate constituents are 
first mixed in the proper proportions and then thoroughly pul- 
yverized. If the raw materials are rocks, the grinding is commonly 


384 THE HYDROUS OXIDES 


carried out in the dry way. On the other hand, soft materials, 
such as marl and clayey mud which are gathered by dredging 
operations, are usually ground wet and are kept suspended until 
dried out in the kiln. 

The burning process is carried out in cylindrical kilns, 100 to 
300 feet in length and 6 to 12 feet in diameter, built of steel plates 
and lined with highly refractory material. The drums are held 
in a slightly inclined position by friction rollers and are rotated 
slowly. The process is continuous, the raw mix entering at one 
end of the kiln and the cement clinker leaving it at the other. 
The heat is derived from pulverized coal, fuel oil, or gas which is 
blown into the lower end of the kiln by compressed air, giving 
a flame 30 to 40 feet in length. The time of passage through the 
kiln is from 1.5 to 2 hours, during which the raw material is 
subjected to a gradually increasing temperature that reaches a 
maximum of about 1425°. In the first stage of the burning proc- 
ess, the raw material is thoroughly dried; in the second stage, 
carbon dioxide and organic matter are driven off; and in the 
final stage, the alumina, silica, and lime react to form the cement 
clinker. The latter consists of partially sintered masses of 
particles from 0.5 to 6 centimeters in diameter. After adding a 
small amount of gypsum which regulates the rate of setting, the 
particles of clinker are ground to a fine powder which is the 
Portland cement of commerce. 

Composition.—The limits of composition within which cements 
of good quality usually fall are set down in Table XX XIII as 
given by Meade.! This, of course, gives only the percentage 
amount of the several components and does not indicate the 
nature of the compounds present. It will be seen that more 
than 90 per cent of the average Portland cement consists of 
calcium, aluminum, and silicon, referred to the oxides; hence, it is 
reasonable to suppose that its properties are due chiefly to 
compounds of these three constituents. As a matter of fact, 
Richardson? demonstrated that a good Portland cement can be 
prepared by starting with lime, silica, and alumina in a pure state. 

Many workers have been concerned with the constitution of 
Portland cement since Le Chatelier published the results of his 


1 “Portland Cement’’ (1925). 
2 Cement, 5, 314 (1904). 


CEMENT 385 


TaBLE XX XIII.—Composition or PortTLAND CEMENT 


Limits of Average 
Constituent composition, | composition, 

per cent per cent 
EE Sy a ts 60.0 to 64.5 62.0 
et ER WE chy os ca ca pi ee ees 20.0 to 24.0 22.0 
NS it i 5.0 to 9.0 7.5 
NS A A 1.0to 4.0 2.5 
PED Seal aoe a 2.0to 4.0 2.5 
ORIG ie hss ek a eee 1. O:too An 75 125 


classic investigations four decades ago. In most of the work, 
the evidence offered in support of the alleged reactions which 
take place during the burning process and of the compounds 
formed is not convincing, since the criteria used to define a 
compound were either indefinite or insufficient. The solution 
of many questions connected with the constitution and setting 
of Portland cement has been brought about by the thorough 
systematic investigations carried out in the Geophysical Labor- 
atory and the United States Bureau of Standards. Thus Rankin 
and Wright? made a complete phase-rule study of the ternary 
system CaQ-Al.0;-SiO2 which necessitated the investigation 
of about 1000 different compositions and fully 7000 heat treat- 
ments and microscopical examinations. The results of these 
observations are summarized in the triangular concentration 
diagram shown in Fig. 30. In this diagram, the pure compo- 
nents are represented by the apices of the triangle; the binary 
mixtures, CaOQ—Al.O3, AlsO3-SiOe, and SiO;—-CaO, respectively, 
by points on the three sides; and ternary mixtures by points 
within the triangle. Each side of the triangle is divided into 100 
parts and all compositions are given as percentage weights of 
the components. ‘The lines within the large triangle divide the 
latter into 14 small triangular spaces which enclose all possible 
mixtures of the three components whose compositions are repre- 
sented by the apices of the respective triangles. The com- 


1“Fxperimental Researches on the Constitution of Hydraulic Mortars” 
(1887), translated by Hall (1905). | 

2Am. J. Sci., (4) 39, 1 (1915); SHEPHERD, RANKIN, and WRicurT: J. 
Ind. Eng. Chem., 3, 211 (1911); Ranxtin: Jbid., 7, 466 (1915). 


386 THE HYDROUS OXIDES 


pounds within each small triangle are represented by symbols 
in which C = CaO, A = Al.O3, and S = SiOs. Thus the 
compound 5CaQO:3Al1.03 is formulated: CsAs. 

Richardson finds that a good cement clinker can be made from 
mixtures of the three oxides in the proportion represented by the 
points at P in Fig. 30. Since all the points lie within the triangle 
whose apices are 8CaO: SiOQo, 3CaO- Al,Os, and 2CaO - SiOz, 





Ce DA 


C3A C5A 3 CA C3A Ls 
Fria. 30.—Diagram showing final products of crystallization of solutions of CaO, 
Al2O3 and SiO». 


it follows that a cement clinker made by burning the three pure 
components in this proportion until equilibrium is reached should 
consist of these three compounds only. If, however, equilib- 
rium were approached but not reached, there should be, in 
addition, only the compounds 5CaO - 3A1,03, and CaO, as these 
are the only other constituents present in the adjacent triangles. 
As will be shown subsequently, observations on commercial 
Portland cement clinker confirm these conclusions. 


CEMENT 387 


As a result of precise investigations of the conditions of forma- 
tion and the optical properties of each of the four compounds 
individually, Rankin! deduces the mode and order of their forma- 
tion in the burning of a cement clinker made up from the pure 
oxides. The first step in the process is the union of lime with 
the other components to give the readily formed compounds, 
5CaO - 3Al.03 and 2CaO - SiOe, probably in this order, since the 
melting point of the former is lower than the latter; subsequently, 
these compounds unite in part with more lime to give 3CaO -- 
SiOz and 8CaO - Al,O;. The formation of the last two compounds 
is a slow process in mixtures of their own composition but is 
hastened in the ternary mixtures by the circumstance that a 
portion of the charge has already melted and acts as a flux or 
solvent. Since 3CaO- S102 is the most important constituent 
in Portland cement, the necessity for burning at a high enough 
temperature to sinter the raw materials is readily understood. 
At a temperature somewhat above 1335°, the conversion of 
5CaO - 3A1,03 to 3CaO - Al,O3? is complete and the important 
compound 3Ca0O - SiOz is forming rapidly. At 1475°, most of 
the lime has entered into combination but a complete melt is 
not obtained until around 1900°. ‘The final products of crystalli- 
zation of this melt are: 3CaO-: SiO, 2CaO- SiOz, and 3CaQO-- 
Al.Os. 

Investigations carried out on commercial Portland cements 
disclose that their composition is essentially the same as the 
clinker made from pure oxides. ‘This is best illustrated by the 
data recorded in Table 34 for (1) pure cement; (2) a commercial 
white cement; and (8) the more common gray variety of com- 
mercial Portland cement. All three are made up largely of the 
same constituents. It is of interest that the optical character- 
istics of the essential compounds persist even when these com- 
pounds are formed in the presence of small amounts of magnesia, 
iron oxide, etc. The magnesia and alkalies are apparently taken 


1J. Ind. Eng. Chem., 7, 466 (1915). 

2 Nore: CamMpsBeEy [J. Ind. Eng. Chem., 9, 943 (1917)] claims that tri- 
calcium aluminate should be regarded either as a metastable saturated solid 
solution of CaO in 5CaO - 3Al1,03 or as 5CaO - 3A1.03 with four molecules of 
CaO of crystallization rather than as a stable phase in the strict sense of the 
word. 


388 THE HYDROUS OXIDES 


up in solid solution by tricalcium aluminate and dicalcium sili- 
cate,! and the iron oxide combines with lime to give ferrite.” 

The minor constituents play an important part in the burning 
of the cement clinker, since their presence results in the formation 
of a flux at a lower temperature, thereby hastening the combina- 
tion of lime with alumina and silica, This is evidenced by the 


TABLE XX XIV.—Compos!ITION OF PORTLAND CEMENTS! 


Relative to | Burning 

content of | temper- | Compounds in re- 

CaO, Al,Os3,-| ature, sulting cements 
SiO, degrees 


Type Actual constituents 











CaO —s 68.4. CaO 68.4 2CaO - SiO, 

SiO. 2a SiO, 23.6 3CaO - Al.O3 

CaO 66.2 | CaO 67.9 2CaO - S102 

Al.Os we 97.6| AlOs 6.5| 1525 3CaO - SiOz 

‘|| MgO, Fe20s, Small-amount of 

Na.O, and eae: ; CaO 

K,O 

CaO 63.2 } CaO 66.7 2CaO - SiOz 

ALO: 7.7} 93:8) Al.Oe 070 [emus 3CaO - SiOz 
Gray. || SiOz _ 22 4] SiO, 24.3 3CaO : Al.Os 

"* |) MgO, Fe2Os, Small amounts of 
Na,O, K,O, 6.7 5CaO Al;Os, CaO, 
and SO3 and ferrites 








1 RANKIN: J. Ind. Eng. Chem., 7, 466 (1915). 


data given in Table XXXIV. It should be pointed out, however, 
that the temperatures are not strictly comparable, since the 
reactions of the white and gray products are incomplete. In 
these cases, it is probable that the temperatures necessary for 
equilibrium would be somewhat higher than the values recorded. 


1 RANKIN and Merwin: J. Am. Chem. Soc., 38, 568 (1916); cf. Kunin and 
Puituips: Highth Int. Cong. Applied Chem., 5, 81 (1912). 

?Sosman and Merwin: J. Wash. Acad. Sci., 6, 15 (1916); CampBELu: 
J. Ind. Eng. Chem., 11, 116 (1919). 


CEMENT 389 


Setting and Hardening.—When finely pulverized Portland 
cement is mixed with water, a plastic mass results which becomes 
solid in the course of a few hours. This process, which is called 
setting, is followed by a gradual increase in strength or hardening 
of the mass. While a good cement becomes very hard in the 
course of a few weeks, it may require years to attain its full 
strength. 

According to Le Chatelier,! the setting and hardening of 
Portland cement consist in the dissolution in water of the anhy- 
drous silicates and aluminates, which subsequently become 
hydrated. Since the hydrates are less soluble than the anhy- 
drous salts, the solutions become supersaturated with respect to the 
former and deposit an entangling mass of needles, thereby giving 
the cement its characteristic hardness. This theory of the 
hardening process was not questioned until Michaelis? recognized 
the formation not only of crystals but of a gel which increased 
gradually until it filled the interstices between the crystalline 
needles as well as those between the cement particles. The 
cementing gel was supposed to be calcium monosilicate and the 
crystals tricalcium aluminate and calcium hydroxide. Accord- 
ing to this hypothesis, the cement particles and crystals become 
embedded in a common sheath of gelatinous substance which 
imparts a degree of hardness that could not be attained by the 
felting of crystalline needles alone. 

More or less successful attempts were made to distinguish 
the various products of hydration of Portland cement by the 
use of organic dyes which stain colloidal and zeolitic minerals 
selectively. Such experiments led Blumenthal* to conclude 
that crystalline monocalcium silicate and tricalcium aluminate 
are among the first products of hydration and that a gelatinous 
silicate forms subsequently. From this, he concludes that the 
setting is due to crystallization alone, the later hardening process 
consisting of the binding together of the crystals and the filling 
of the pores by means of a gel. 


_ 1xperimental Researches on the Constitution of Hydraulic Mortars” 
(1887), translated by Hall (1905). 
2 Kolloid-Z., 5, 9 (1909); 7, 320 (1910); Chem. Ztg., 17, 982 (1893). 
3’ KEISERMANN: Kolloidchem. Bethefte, 1, 423 (1910). 
4 Silikat Z., 2, 43 (1914); Thesis, Jena (1912). 


390 THE HYDROUS OXIDES 


A systematic investigation of the setting and hardening process 
was possible only after the constitution of cement had been 
definitely established. Knowing that cement consists essentially 
of 3CaO- Al,O3, 8CaO-SiOs, and 2CaO- 8iOs, investigations 
were carried out by Klein, Phillips, and Bates! in the U. 8. 
Bureau of Standards to determine what happens when each of 
the constituents separately is brought in contact with water. 
The results of these and later observations are as follows: 

Tricalcium Aluminate—When 3CaQO: Al.O3 is mixed with 
water, a gelatinous hydrous material is first formed which sets so 
rapidly that it is almost impossible to make test pieces. It 
never attains a tensile strength much beyond 100 pounds per 
square inch. When mixed with silicate, it affects the latter more 
markedly in the time of set than in the strength, tending to 
hasten the former and retard the latter. With the limited amount 
of water in cement pastes, it is converted into and remains a gel 
during the first 24 hours, at least.2, With more water, crystalliza- 
tion takes place fairly rapidly. Pulfrich and Linck? isolated 
crystals having the composition 3CaO- Al,O3;:7H2O, which 
were claimed to be identical with those in cement. Duchez* 
claims that all calcium aluminates in cement hydrate to 3CaO -- 
Al,O;: 12H.O. If lime aluminates of lower basicity are present, 
a quantity of hydrous alumina is liberated which in turn combines 
with Ca(OH). from the hydrolysis of calcium silicates, forming 
the duodecahydrate. 

It is an interesting fact that 3CaO - Al,O3, the single aluminate 
present in Portland cement, is the only one that does not possess 
hydraulic properties.» This is probably because the action of 
water on aluminates other than 3CaO- Al.O3; gives some 
hydrous 3CaO - Al,O3, which later crystallizes, and gelatinous 
alumina which is the real cementing agent in aluminous cements. 

Dicalcium Silicate—The compound 2CaO: SiO» sets very 
slowly with water, and it is only after long intervals that sufficient 

1KuEIn and Puitups: U. S. Bur. Standards, Techn. Paper 43 (1914); 
Batss and Kern: [bid., 78 (1916). 

2 Bates: J. Am. Ceram. Soc., 2, 708 (1919). 

3 Kolloid-Z., 34, 117 (1924). 

4 Rock Products, 27, No. 18, 62 (1924). 


5 Bates: J. Am. Ceram. Soc., 1, 679 (1918); U. S. Bur. Standards, Techn. 
Paper 197 (1921). 


CEMENT 391 


gelatinous material is produced to cement the granules of com- 
pound together into a solid mass. After 14 days, a test piece 
broke at less than 60 pounds per square inch; but at the end of a 
year, a sample developed a tensile strength of 600 pounds per 
square inch. The cementing material is hydrous monocalcium 
silicate, which, together with calcium hydroxide, is formed by 
the slow hydrolysis of the silicate. 

Tricalcium Silicate-—3CaO - SiO» is the only one of the three 
major constituents which reacts with water within a reasonable 
time to give a mass comparable to Portland cement in hardness 
and strength. The end products of the reaction are hydrous 
monocalcium silicate and calcium hydroxide, as in the case of 
dicalcium silicate; but unlike the latter, tricalcium silicate 
hydrolyzes very readily to give the essential binding gel and so is 
the most important constituent in Portland cement. 

Summary.—When water is added to a mixture of the three 
constituents as they occur in Portland cement, the initial set is 
due to the formation of a gel of tricalcium aluminate and possibly 
a small amount of monocalcium silicate. Pulfrich and Linck? 
emphasized the importance of gel formation in the initial stage 
by showing that crystallization does not take place at the outset 
in the presence of the amount of water used in technical practice. 
Their observations were made in glycerin solutions in order to 
get the necessary dilution for microscopic examination; and the 
glycerin may have inhibited the crystallization. This, however, 
merely emphasizes the contention that setting is not necessarily 
occasioned by the formation of microscopically visible crystal 
needles. It is true, of course, that some crystallization takes 
place in time; but the only crystalline bodies which form ordina- 
rily? are calcium hydroxide* and a crystalline hydrate derived 
from tricalcium aluminate, probably the duodecahydrate. 

Whereas the initial set results primarily from the formation of 
tricalcium aluminate gel, the subsequent fairly rapid increase in 


1 MicnaeEtts: Kolloid-Z., 5, 9 (1909); 7, 8320 (1910); DucuEz: Rock Prod- 
ucts, 27, 18, 62 (1924). 

2 Kolloid-Z., 34, 117 (1924). 

3 Cf., however, GLASENAPP: Zement, 11, 446 (1922); Ktun: [bid., 138, 362, 
375 (1924.) 

4 Cf. Barkov and Racozinski: J. Russ. Phys.-Chem. Soc., 47, 761 (1916). 


392 THE HYDROUS OXIDES 


cohesive strength and hardness is due in large measure to the 
liberation of hydrous monocalcium silicate by the hydrolysis 
of the tricalcium silicate and dicalcium silicate. It is a pity 
that dicalcium silicate does not hydrolyze more rapidly, for it is 
formed at a lower temperature than tricalcium silicate, and yields 
ultimately a higher percentage of the important binding material, 
monocalcium silicate. 

As already noted, gypsum is added to cement clinker before 
erinding, in order to retard the time of set. The gypsum may 
function by diminishing the solubility of tricalctum aluminate or 
by precipitating calc1um sulfoaluminate,! thus removing lime 
from solution which would otherwise be available for the forma- 
tion of tricalcium aluminate gel. 

Tippermann? is of the opinion that the presence of gypsum 
serves two functions; it retards crystallization and aids the 
formation of colloids. This opinion is based on the observation 
that sulfate-free cements to which no gypsum is added, undergo 
rapid and extensive crystal formation, but no colloidal material 
is present at the end of a year. The addition of gypsum to such 
cements cuts down the rate of crystallization; but swelling, 
together with gel formation, takes place at once. Tippermann 
attributes the action of gypsum to the sulfate ion and not to 
calcium ion, since in the sulfate-free cement, the concentration of 
calcium ion varies from zero to saturated solution without gel 
formation entering in. ‘These observations should be confirmed 
and extended. 

Since gypsum is softer than clinker, it is probable that the 
gypsum particles are ground considerably finer than the cement 
particles. It has been suggested that the more finely divided 
gypsum particles coat the coarser cement particles, thereby 
~ acting to some extent as a protecting film and so delaying the 
chemical process involved in setting.*® 

The addition of salts influences the time of set to a greater or 
lesser degree. Gadd‘ reports the results of recent observations 
with a large number of compounds including the carbonate, 


1 Ktuu: Prot. ver. D. P. C. F., 45, 98 (1922). 

* Zement, 18, 1385, 147 (1924). 

3 Fink: J. Phys. Chem., 21, 32 (1917); Briaes: Ibid., 22, 216 (1918). 
4 British Portland Cement Research Assoc., Pamphlet 1 (1922). 


CEMENT 093 


nitrate, chloride, sulfate, borate, and hydroxide of sodium, 
ammonium, aluminum, zinc, cobalt, and chromium. Of the 
various compounds studied, the nitrates appear to have little 
effect on the rate of setting, whereas all the other compounds 
except gypsum and plaster of Paris accelerate the set. In view 
of the influence of electrolytes on jelly formation,! it is not sur- 
prising to find that their presence has an effect on the rate of 
setting of cement. One would expect the addition of salts?® 
to have either a retarding or accelerating action, depending on 
whether they have a coagulating or stabilizing action on the 
colloids formed by the action of water on the cement particles. 

The addition to cement of calcium chloride or ‘‘Cal’’* materi- 
ally accelerates the rate of hardening of Portland cement mix- 
tures. This is probably due to the precipitation of a calcium 
chloraluminate of the composition 3CaO - Al,O3- CaCl,: 18H.0,° 
with an accompanying decrease in the pH value. This reduction 
in pH accelerates the hydrolysis of the silicates and so hastens the 
hardening process. Platzmann,® on the other hand, attributes 
the action mainly to the hygroscopicity of calcium chloride which, 
by absorption of moisture during the first few weeks, prevents 
the shrinking and cracking of the cement and protects it from 
too rapid a loss of moisture. 


CEMENTS RELATED TO PORTLAND CEMENT 


Iron-Portland Cement.—A cement may be prepared in which 
iron is substituted for aluminum. It is manufactured in much 
the same way as ordinary Portland cement; and as in the latter, 
the chief hydraulic constituent is tricalcrum silicate. The aréa 
occupied by cements rich in iron oxide in a triaxial diagram of 
the system lime-silica-iron oxide is in nearly the same position as 


1 See p. 26. 

2 RoHLAND: Kolloid-Z., 8, 251; 9, 21 (1911). 

3 Cf. Benson, NEWHALL, and Trempsr: J. Ind. Eng. Chem., 6, 795 (1914). 

4 A material resulting from the interaction of lime and calcium chloride in 
water. 
6Laruma: “Le Ciment,’’ 174 (1925); cf., however, Kijtu~t and Unricu: 
Zement, 14, 859, 880, 898 (1925); GassnurR: Chem. Ztg., 48, 157 (1924). 

6 Zement, 10, 499 (1921); 11, 137 (1922); Chimie & industrie, T, 943 (1922); 
8, 614 (1922). 


394 THE HYDROUS OXIDES 


that of Portland cement in the system lime-silica-alumina.! 
Iron-Portland cement with its lower lime content” contracts more 
on setting than does Portland cement. The addition of calcium 
chloride to iron-Portland or blast-furnace cements causes them 
to swell, so that the natural shrinkage is counteracted or takes 
place only after a long time. 

In Germany, up to 30 per cent of blast-furnace tae is added to 
Portland cement clinker giving what is called Eisen-Portland 
cement. This produces a superior product for sea-water con- 
struction, possibly because the added slag unites with any free 
lime, thereby preventing it from acting with the sea water to 
form calcium hydrosilicates* or such compounds as magnesium 
hydroxide® or calcium sulfoaluminate,® which are active in pro- 
ducing cracks. 

Aluminous Cement.—Cements in which the alumina content is 
equal to or greater than that of the silica content are known 
commercially as ‘‘aluminous,” ‘‘fused,”’ or ‘‘electrofused”’ 
cements. They are produced by fusion, because calcium alumi- 
nates soften readily, and clinkering is very difficult.’ As pre- 
viously mentioned, Bates* found that all the alumina compounds 
in the lime-alumina-silica system possess hydraulic properties 
except 3CaO- Al.O3. The compound 5CaO - 3Al.03 sets very 
rapidly indeed; while both 3CaO - 5Al.03 and CaO - Al.O3 set 
slowly but harden rapidly, developing great strength in 24 hours. 
Very good cements may be had with 55 to 75 per cent alumina in 
lime-alumina burns. Aluminous cements are manufactured 
extensively in France; but the chief drawback to their wide com- 
mercial use is the lack of a widely distributed supply of hydrous 
alumina and the consequent high cost of raw materials. 


1 Kin: Zement, 10, 361, 374 (1921). 

2 CAMPBELL: J. Ind. Eng. Chem., 11, 116 (1919). 

3 GUTTMANN: Zement, 9, 310, 429 (1920). 

4Cf. GassneR: Practical Questions Concerning Concrete in Sea Water, 
Zement, 14, Nos. 21 to 25 (1924). 

5 Lewis: Engineering, 109, 626 (1920); Gary: Mitt. Material-prifungsamt, 
37, 12 (1919). 

6 Grtn: Zement, 12, 297, 307, 317, 326 (1924). 

7 Brep: Techn. ees 14, 508 (1922); Rev. métal., 19, 759 (1922). 

8 J. Am. Ceram. Soc., 1, 679 (1918); U. S. Bur. Standards, Techn. Paper 
197 (1921); cf. ENDELL: Zement, 8, 319, 334, 347 (1919). 


CEMENT 395 


The setting and subsequent hardening of aluminous cements 
result from the formation of tricalcium aluminate gel and hydrous 
alumina.! ‘The early hardening is due to the relatively high rate 
of hydrolysis of the aluminates. Advantages claimed for 
aluminous cement over Portland cement are: the more rapid rate 
of hardening; greater strength; and the higher temperature 
developed on setting, usually sufficient to permit normal harden- 
ing even in severe weather.’ 


1 Ktuu and Tuurine: Zement, 18, 109, 243 (1924); PLatzmann: Rock 
Products, 27, No. 19, 23 (1924). 

2 GUERITTE: Contract Record, 38, 1197 (1924); Anon.: Eng. News-Record, 
94, 320 (1925). 


CHAPTER XIX 
THE SOIL 


In his classic work on adsorption by the hydrous oxides, van 
Bemmelen! advances the idea that the inorganic colloids in the 
soil are similar in general nature to the gelatinous oxides. ‘This 
idea has persisted, and according to Whitney :? ‘‘It is now coming 
to be quite generally believed that the inorganic colloidal material 
of the soil is essentially the same as the artificial gels of silica, 
iron, and alumina, which have been prepared.” ‘There are, 
however, a number of people? who champion the view that a con- 
siderable portion of soil colloids consists of complex acid alumino- 
silicates rather than a mixture of the hydrous oxides. In any 
event, it would seem that a volume devoted to the hydrous 
oxides would be incomplete without some reference to the col- 
loidal matter of the soil. 


COMPOSITION OF THE SOIL COLLOID 


The colloidal matter of the soil is derived from both organic 
and mineral sources. The organic colloidal matter consists of 
the remains of animal and vegetable life, together with the soil 
bacteria and fungi. In such organic soils as the so-called peats 
and mucks, the colloids are chiefly organic; but in most agricul- 
tural soils, the colloidal matter is of mineral origin, derived in 
large measure from the hydrolysis of silicates. 

It is difficult, if not impossible, to separate all the colloidal 
matter from a soil. The earlier investigators merely rubbed up 
the soils with a considerable amount of water and estimated as 
colloidal matter the amount that remained suspended for a 

1 “Die Absorption,” 114 (1910); LANDER: Ber. Stat., 28, 265 (1879). 

2 Bogue’s ‘‘Colloidal Behavior,” 2, 468 (1924). 

3SHarP: Univ. Calif. Publ. Agr. hie 1, 291 (1916); BRADFIELD: Colloid 
Symposium Monograph, 1, 369 (1923); a oe Soc. Agron., 17, 253 (1925); 
Truoa: Colloid Symposium Monograph, 3, 228 (1925). 

396 


THE SOIL 397 


given length of time. Schlésing! is of the opinion that the 
material which remains longest in suspension differs essentially 
from material which does not remain suspended so long and so 
estimates the colloid content of soils to be only 0.5 to 1.5 per 
cent.? Hilgard* and Williams‘ reported much higher percentages 
based on the amount of material that does not settle in a 24-hour 
period. Since the amount of soil that will remain suspended 
depends on the degree of peptization of a gel and the time of 
settling, methods of estimating the colloid content of soils based 
on such procedures® are necessarily inaccurate. Other methods 
that have been employed are based on determination of the 
adsorption capacity of the soil for malachite green,® water, and 
ammonia. Gile’ and his coworkers determine the adsorption 
capacity of a sample of soil and of the colloidal material extracted 
frem the soil, and from these data, calculate the percentage 
colloidal matter. After correcting for the possible alteration in 
adsorptive capacity of the colloid produced by extraction, the 
percentages of colloidal matter indicated by adsorption of mala- 
chite green, water, and ammonia show fairly good agreement 
among themselves® and with the percentages estimated gravi- 
metrically and microscopically. As would be expected, the 
colloidal content of different soils varies widely. Assuming that 
all particles less than lu in diameter are colloidal, the sandy 
soils contain but a few per cent of colloids; while the loam soils 
may contain 15 to 25 per cent, and the ote 40 to 50 and up to 
90 per cent colloidal matter. 

The method of procedure employed in the Bureau of Soils, 
U. 8. Department of Agriculture, for separating samples of 
colloids from the rest of the soils is essentially as follows: The 
soil is suspended in distilled water or in water containing enough 
ammonia to impart a pH of 7 to 8. After allowing to settle for 


1 Compt. rend., 70, 1345, 1870; 78, 1276; 79, 473 (1874). 

2 Cf. EHRENBERG: ‘‘ Die Bodenkolloide,’”’ Dresden, 99 (1922). 

3 Am. J. Sci. Arts, (3) 106, 288, 333 (1873); ‘Soils,’ New York, 333 (1919). 

4 Forsch. Gebiete Agrikuitur-Physik., 18, 225 (1895). 

5 ScaLtes and Marsu: J. Ind. Eng. Chem., 14, 52 (1922). 

6 Asutey: U.S. Geol. Survey Bull. 388, 65 (1909). 

7 Gite, MrppLeTon, Rosinson, Fry, and AnprERSON: U. S. Dept. Agr. 
Bull. 1193 (1924). 

8 Cf. Davis: J. Am. Soc. Agron., 17, 277 (1925). 


398 THE HYDROUS OXIDES 


18 hours, the turbid supernatant liquid is passed through a 
supercentrifuge where each particle is exposed to a force of 
approximately 17,000 gravity for 3 minutes. The colloid which 
passes through the supercentrifuge is collected on the outside of a 
Pasteur-Chamberlain filter by sucking off the water. The 
average diameter of the particles obtained by this procedure 
is 0.1 to 0.15u, the largest being about 0.3u. The residue 
appears distinctly gelatinous and dries to a hard, horn-like mass. 

To give some idea of the composition of the soil colloids, there 
are given in Table X XX V the analyses of anumber of such colloids 


TABLE XXXV.—ComMPOSITION OF Soin CoLLoIDS 


Soil type 
Substance 
‘|? |) 4) 5) oor 
iLO Pes Se gere nw 5 / 50.49 |50.13 |44.94 |48.04 |42.40 /36.26 [81.30 |15.86 
Wee ea eee 0.51 | 0.46 | 0.47 | 0.65 | 0.56 | 0.65 | 1.01 | 3.54 
AloO3..........{16.73 |21.70 |22.15 |25.19 |24.71 (32.85 |33.64 |34.38 
Hee cave ei 10.77 | 8.70 | 8.91 | 8.80 {15.27 |12.44 |11.66 (22.67 


MnO ee 0.121 
(OA Me ee ae 2.00 
NipO siete 5-82 |-2:54-1) 120506458 
HO eee eae 2.24 86402507 sis ese 


0.035} 0.126) 0.032) 0.138} 0.160; 0.070) 0.068 
1 
2 2 
1 2. : 
Na,O...........| 0.54 | 0.24 | 0.19 | 0.88 | 0.51 | 0.47 | 0.58 | 0.33 
0 0 
3 3 
8 7 


.48 | 1.12 | 1.29 | 1.18 | 0.44 | 0.56 | 0.21 


Jae As ter 0.37 09: 120707 7051s 
Organic matter.| 1.79 .83 | 7.94 | 4.52 
9 


Combined H,0.| 8.26 wf gto ron .23 .22 |12.73 |11.79 |15.63 


. Ontario loam, subsoil, New York 

. Vega Baja clay loam, soil, Porto Rico 
. Cecil loamy fine sand, soil, Georgia 

. Aragon clay, deep subsoil, Costa Rica 


1. Fallon loam, soil, Nevada 

2. Sharkey clay, soil, Mississippi 

3. Marshall silt loam, soil, New York 
4. Carrington loam, subsoil, Iowa 


CONDON 


from widely different types of soils. This table was compiled 
from data obtained in the Bureau of Soils of the U. 8. Department 
of Agriculture.t Although the composition of the colloids from 
different sources may show wide variation, these differences may 
be relatively small in soils from similar climatic regions.” Inves- 
tigations on a large number of representative soils disclosed 

1Cf. Gite: Colloid Symposium Monograph, 3, 218 (1925); Rosrnson and 


Houmss: U.S. Dept. Agr. Bull. 1311 (1924). 
2 BRADFIELD: J. Am. Soc. Agron., 17, 253 (1925). 


THE SOIL 399 


that the composition of the colloids, as compared with that of 
the whole soil, was much higher in alumina, ferric oxide, organic 
matter, water, magnesia, phosphorus, and sulfur, and lower in 
silica. In the ageing process to which the soil is subjected, there 
appears to be a tendency either for the silica to be transformed 
into secondary quartz or to move below the soil layer, while iron 
oxide and alumina accumulate in the soil colloids. 

The colloidal mineral matter of soils appears to be formed by 
the action of water on hydrated silicates of igneous origin. 
Whitney! believes the soil colloids are formed by the bombard- 
ment of soil particles by water molecules when the former are 
of the order of magnitude of 0.0001 millimeter in diameter; 
while Gordon? considers that the outer layer of all silicate parti- 
cles is constantly subjected to hydrolytic action. By these 
weathering influences, there are formed the insoluble hydrous 
oxides of iron, aluminum, and silicon, and the soluble salts of 
sodium, potassium, calcium, and magnesium, which are adsorbed 
in part by the hydrous oxide gels. There appears to be no con- 
clusive evidence as to whether the hydrous oxides remain as such 
in the soil, retaining more or less of the adsorbed soluble constitu- 
ents, or whether in the course of time there are formed complex 
aluminosilicates of definite composition. On account of the 
variability in the proportion of alumina to silica, it is obvious 
that no one fixed proportion would account for all the alumina or 
all the silica in every colloid. It is necessary, therefore, either to 
postulate the existence of several complex silicates or to assume 
that a portion of the hydrous oxides remain as such. Until 
the question is definitely settled, I subscribe to the simpler 
assumption that the inorganic soil colloids consist essentially of 
variable amounts of the hydrous oxides of iron, alumina, and 
silica with varying amounts of adsorbed salts. For the most 
part, the so-called aluminosilicates are adsorytion complexes of 
indefinite composition, formed by the mutual precipitation of 
negatively charged hydrous silica and positively charged hydrous 
alumina. It is probable that the organic material* and the 

1 Science, 64, 656 (1921). 

2 Science, 55, 676 (1922). 

3’ EHRENBERG: Z. angew. Chem., 41, 2122 (1908); Wreaner: Kolloidchem. 
Bethefte, 2, 238 (1910); Fopor and ScuHornretp: Kolloidchem. Bethefte, 
19, 1 (1924). ) 


400 THE HYDROUS OXIDES 


hydrous silica keep the colloidal soil material in a dispersible state. 
As Gile! put it: “‘In most soils, colloidal material has probably 
persisted several thousand years, undergoing some changes, but 
remaining nevertheless a dispersible colloid. The experiment 
will never be performed; so it is safe to predict that pure inorganic 
gels would not preserve their characteristics over the same period 
of time.” Bradfield? points out that mixtures of artificial 
colloids having the same composition as colloids found in the soil 
do not have the same properties as the soil colloids. This, in 
itself, offers no proof that the soil colloids do not consist essen- 
tially of the hydrous oxides of iron, aluminum, and silicon together 
with colloidal organic matter and adsorbed salts. It would 
seem impossible to prepare a synthetic soil that even approaches 
the properties of a true soil until one can duplicate very closely 
the conditions of formation of the hydrous oxides and the influence 
of salts, organic matter, and other soil conditions which enter 
into soil formation. 

The organic colloids introduced into the soil in the form of 
plant and animal residues are subjected to the action of bacteria 
and other lower forms of life which cause porfound changes. 
Under aerobic conditions, that is, under conditions of good 
aeration such as exist in cultivated soils, the organic matter is 
oxidized fairly completely, giving water and carbon dioxide and 
the phosphates, carbonates, nitrates, and sulfates of sodium, 
potassium, calcium, and magnesium which are made available 
for new plant growth. On the other hand, under anaerobic 
conditions such as obtain in poorly drained and hence poorly 
aerated marshes and swamps, a part of the organic matter is 
decompsoed with the formation of a colloidal substance known 
as humic acid or humus. ‘This product is a dark, waxy mixture 
of many complex compounds.? 

-The so-called humic acids are formed also in drained prairie 
soils which are covered with a dense growth of grass. The sod 
provides what amounts to partial anaerobic conditions by pre- 
venting rapid aeration, so that dead roots, stems, and leaves of 
grass are in part converted into humic acid. In the presence of 


1 Colloid Symposium Monograph, 3, 227 (1925). 
2 Missouri Agr. Exp. Sta., Res. Bull. 60 (1923). 
3 SCHREINER and SuHorey: U.S. Bur. Soils Bull. 74 (1910). 


THE SOIL 401 


considerable calcium, the organic matter may be quite black. 
The dark color of prairie soils is probably due to a coating of 
the black organic substance on the particles of mineral matter. 
After the humus substance is once formed, it resists decomposi- 
tion when exposed to aerobic conditions, as evidenced by the 
fact that the cultivation of black prairie land for many years 
does not cause it to lose its black color. 

The bacteria and other living organisms constitute a very 
important part of the colloidal matter of the soil; but they repre- 
sent a very minute proportion of the total weight. 


RELATION BETWEEN PROPERTIES AND COMPOSITION OF SOIL 
COLLOIDS 


Although colloidal soil material contains a number of constitu- 
ents in variable proportions, the three major constituents in the 
colloids from soil other than peat soils are silica, alumina, and 
ferric oxide. As a result of investigations carried out in the 
U. 8. Bureau of Soils, it has been demonstrated that the prop- 
erties of soil colloids vary fairly regularly with the contents of 
the major constituents as expressed by the molecular ratio of 
silica to alumina plus ferric oxide. This is well illustrated in 
Table XXXVI, compiled by Anderson and Mattson.? In this 
table, a series of colloids extracted from different soils is arranged 
in ascending order of the molecular ratio, silica to alumina plus 
ferric oxide. In columns 3 and 4, respectively, are given the 
heats of wetting in calories per gram of colloid and the amounts 
of ammonia gas adsorbed per gram of colloid. In columns 4, 5, 
and 6, the data of 2, 3, and 4 are expressed relatively, in order to 
make the relationships more apparent and to bring out individual 
exceptions. The evidence indicates that the correlation between 
heat of wetting and ammonia adsorption will hold fairly well 
for practically all soil colloids,* whereas the relationship between 
the molecular ratio of silica to alumina plus ferric oxide and the 


1 ANDERSON: J. Agr. Research, 28, 927 (1924); Grin, et al.: U. S. Dept. 
Agr. Bull. 1193 (1924); Rosinson and Houtmgs: U. S. Dept. Agr. Bull. 
1131 (1924). 

2 Science, 62, 114 (1925). 

3 Cf. Bouroucos: Soil Science, 16, 320 (1924). 


402 THE HYDROUS OXIDES 


TABLE XXX VI.—RELATION BETWEEN COMPOSITION AND PROPERTIES OF 
Sort CoLLoips 





Actual values for Relative values for 





Source of 


colloidal material Ps Heat of | NHsad- fed. 


wetting, | sorbed, 


Heat of | NH3;ad- 
Ale O3 + FeO; | calories | grams | AleO3 + Fe2O3 


wetting | sorbed 





Cecil subsoil...... 1.20 4.5 0.0192 0 0 3 
Cecil soil... .a<c.<. 1.34 6.2 | 0.0230 ae 13 12 
Chester soil....... ii ee tee 0.0293 28 21 26 
Norfolk subsoil... 1.84 6.0 0.0295 oe TL 27 
Huntington soil... 1.86 8.3 0.0319 33 29 32 
Sassafras subsoil. . 1.89 9.8 0.0340 34 40 37 
Hagerstown sub- 

BOL tienes ederecets 1.89 729 0.0299 34 26 28 
Susquehanna sub- 

BOL e ere eters 1.98 ee} On0LT 7 40 6 0 
Miami subsoil.... 2.66 11.8 0.0358 2 56 41 
Marshall soil..... 2.82 14.2 0.0536 80 74 82 
Stockton soil..... 2.85 16.3 0.0617 81 90 100 
Wabash soil...... 3.16 17.6 0.0614 97 100 99 
Sharkey soil...... 3.20 16.3 0.0609 100 90 98 





heat of wetting or ammonia adsorption is subject to some marked 
exceptions. Thus, the Susquehanna subsoil shows good agree- 
ment between heat of wetting and ammonia adsorption, and poor 
agreement between silica ratio and either heat of wetting or 
ammonia adsorption. 

A fairly good correlation has also been found! to exist between 
the ratio Gs eR: and the ratio ieee ee for a 
series of soils. It thus appears that chemical constitutents other 
than those which enter into the silica ratio correlate with the 
properties of the soil colloids. As an example, some properties 
appear to be closely related to the percentage of calcium or to 
the total exchangeable monovalent and bivalent bases. 

Such information as contained in Table XXXVI on the rela- 
tionship between composition and properties of the soil colloids is 
of practical value in enabling one to predict qualitatively the 
general behavior of the colloids without making complete chem- 
ical analyses or extensive physical tests.? 








1 Ropinson and Hoimes: U. 8. Dept. Agr. Bull. 1311 (1924). 
2 Cf. ANDERSON and Marrtson: Science, 62, 114 (1925). 


THE SOIL 403 


Tur ROuE or THE Sort CoLLoIps 


_ Adsorption of Salts.—On accuont of the high specific surface 
of the colloidal material in the soil, it is the colloids chiefly which 
adsorb certain mineral constituents, especially the so-called 
plant foods, and yield them up to growing plants as required. 
This is illustrated by some observations of Gordon! on the adsorp- 
tion of calcium acid phosphate by the hydrous oxides of silicon, 
iron, and aluminum, Table XXX VII. Sulfate and nitrate are 
also adsorbed, but less strongly than phosphate. Since the 


TABLE XX X VII.—ApDSORPTION OF CaLcIuM AcID PHOSPHATE BY HypROUS 
OXIDES 
Milligrams Ca eeetbes per gram | Milligrams POs adsorbed per gram 
of ge 


Concentration of of gel 





solution A j Ferric “A oe 

Silica Alumina as Ps Silica Alumina eatde 

‘twa eee 0.12 | 84.9 | 121.4 | 0.08 | 610.2 | 609.9 
1 ee ok ae 0.12 51.4 83.0 0.04 393.4 421.6 
Ber SAR es ce 0.11 32.7 54.5 0.01 2726 266.6 


amount adsorbed is in equilibrium with a solution of certain 
concentration, leaching with water should remove a portion of 
the adsorbed salt. Gordon finds that 40,000 cubic centimeters of 
water must be passed through the gel before the filtrate fails to 
give a test for phosphate. By this procedure, most of the 
adsorbed salt is leached out from the particular sample of silica; 
but the alumina and ferric oxide still retain a large portion of the 
salt, as shown in Table XX XVIII. It thus appears that below a 
certain concentration, rain will leach out but very small quanti- 
ties of phosphate from a soil made up of hydrous oxides. To 
determine whether salts which can be removed only in extremely 
small amounts by leaching are available for plant growth, Gordon? 
prepared a synthetic soil by mixing the leached hydrous oxides 
with pure quartz sand and used this in investigations on plant 
growth. Unmistakable evidence was obtained that plants 
derive all the phosphorus required for their nourishment from the 


1 LICHTENWALNER, FLENNER, and Gorpon: Soil Science, 15, 157 (1923). 
2 WiuEY and Gorpon: Soil Science, 15, 371 (1923). 


404 THE HYDROUS OXIDES 


TABLE XXX VIII.—Errect or LEACHING ON THE PHOSPHATE CONTENT OF 
GELS 


Milligrams PO, adsorbed per gram of 


Ferric oxide Alumina 





Before, washine...s.. wiotes = eee 25.9 162.4 
After washing s,s cca toes ee 16.4 117.5 


hydrous oxides containing adsorbed phosphates. Apparently, 
the roots of the plant in intimate contact with the colloidal oxides 
take up the very slight equilibrium concentration of soluble 
phosphorus which is replaced continuously. Obviously, the 
equilibrium concentration decreases as the amount of adsorbed 
phosphate in the soil decreases, so that plants are not well nour- 
ished if the phosphate content is too low. As Gordon points out, 
it is immaterial whether the plants take up the phosphate from 
CaH4(PO.). or from some complex compound; the concentration 
of salt in equilibrium with the adsorbed phosphate determines the 
amount available at a given time. 

Nitrogen in the form of nitrate is preferred by most plants; but, 
as already noted, the element cannot be stored as nitrate since 
the latter is so weakly adsorbed by the soil colloids that it is 
readily leached out and lost. The original source of nitrogen for 
plants, other than the legumes, is organic matter which is changed 
by the action of bacteria into ammonia and subsequently into 
the relatively strongly adsorbed ammonium salts. The latter 
transformation is a result of the soil acidity which is derived in 
large measure from the hydrolysis of salts and the relatively 
stronger adsorption of base than of acid by the soil colloids. 
Under the influence of bacteria, the small equilibrium concen- 
tration of ammonium salt is oxidized slowly to nitrate and 
becomes available for plant food. 

Soluble potassium salts are sufficiently strongly adsorbed! 
that they are conserved in the soil and are gradually given up to 
plants as needed. The relative adsorbability by hydrous oxides 
of the ions from solutions of sulfates and of the primary phos- 


1 BoauE: J. Phys. Chem., 19, 665 (1915). 


THE SOIL 405 


phates of potassium magnesium and calcium is recorded in 
Table XX XIX, compiled from data by Gordon.! It will be 
seen that the adsorption is similar to that of calcium and con- 
siderably stronger than that of magnesium. The last two rows of 


TABLE XX XIX.—ApsorPTION OF Ions BY Hyprous FERRIC OXIDE AND 
















































































ALUMINA 
Milligrams ions adsorbed per gram Fe203 | Milligrams ions adsorbed per gram Al2O3 
KH2PO.4 | Meg(HePO.)2 | Ca(H2PO.1)2 | | Meg(H2PO.). | Ca(H2PO.)2 
K | PO; Mg | PO; Ca PO. | Mg PO, Ca PO, 
43.8 U6aced esi) 165.0 | 50.7 | 235.6 14.0 | 165.0} 47.0 | 239.0 
K.2SO: | MgSO. | CaSO. | K2SO4 | MgSO, | CaSO. 
K | SO; Mg | SO, Ca | SO.4 K | SO. Mg | SO, Ca SO4 
11.4 13.6 feos eae te 12.9 +) 36.09) 11.0) 19.0 Gaze ors atte Wh UPA 
29.0 mietee (cell Weer 20 8 24.0 16 apes pn 13.2 


figures represent the adsorption of ions before adding phosphate 
and after adding phosphate, respectively. The more strongly 
adsorbed phosphate ion displaces completely the sulfate ion and, 
at the same time, increases greatly the adsorption of the cations. 

Using a sandy loam soil as adsorbent instead of the pure hydrous 
oxides, Harris? obtained the results recorded in Table XL. 


TABLE XL.—ADSORPTION OF CATIONS BY A SOIL 


| Adsorption of cation by 50 grams of soil 


Solution | ss 
| Grams Equivalents 
DLO PS oR 0.113 0.00125 
UNS Geo See A Se 0.0395 0.00101 
eee oe es ese 0.01384 0.00067 
A ee 0.0177 0.00064 
IMEC On 2 ae 0.0057 0.00047 
CT as ee en 0.0041 0.00013 





1 LICHTENWALNER, FLENNER, and Gorpon: Soil Science, 15, 158 (1923). 
2 J. Phys. Chem., 21, 454 (1917). 


406 THE HYDROUS OXIDES 


In these experiments 50-gram samples of soil were shaken 
frequently with 125-cubic-centimeter portions of the salts during 
a period of 24 hours. The salts are arranged in the order of 
adsorbability of the cations. It will be seen that potassium ion 
is adsorbed more strongly than the divalent ions and almost as 
strongly as trivalent aluminum. The adsorbed potassium ion is 
displaced by other ions as given in Table XLI. The soil was first 
treated with potassium chloride, thoroughly washed, and dried, 
after which 50-gram portions were treated for 72 hours with the 


TABLE XLI.—DiIsPLacinG or Potassium ION FROM SOIL BY OTHER CATIONS 


Grams of potassium ion 
Fifty-gram samples treated with 200 
cubic centimeters of distilled water Found in 


solution Diepiaces 

Distilled water... oc. ped see a ee Py 0.0076 

NJ10 AICIs be nG P0S% ee 0.0512 0.0436 
N-/1O INH ies eee ee ee eee 0.0471 0.0395 
NF10 Min Clo.i' 200 wadnaesl A cthe Ga ee 0.0426 0.0350 
NJ10: Cale. «icine nyt nen as ties ten 0.0424 0.0348 
Water containing 1.2 grams CaSO, -2H.O.... 0.0423 0.0347 
N/10' MgeClac es eres eee Cle 0.0343 0.0267 
N10 NaCl a Gere. ee 0.0326 0.0250 


solutions listed in the table and the adsorption measured. As 
would be expected, the order of the displacing power of the cations 
is approximately the same as the order of adsorbability given in 
Table XL. Particular attention should be called to the strong 
adsorbability of ammonium ion as evidenced by a Cee 
power second only to that of aluminum ion. 

Thus, we may look upon the inorganic and organic colloids as a 
reservoir which adsorbs! and so conserves the plant foods and 
yields them up as needed to the growing plants. Another 
point of view is that the soil colloids are amphoteric compounds 
which bind the mineral constituents as definite complex salts 
possessing the necessary solubility to supply food to the plants as 
required. Until it has been proved that the inorganic soil col- 


1ScuHié6sina: Ann. chim. phys., 5, 2 (1874); ScHREINER and FAILYER: 
U.S. Bur. Soils Bull. 32 (1906). 


THE SOIL 407 


loids are largely complex silicates which combine with the so- 
called plant foods, yielding complex salts with all the necessary 
properties, I prefer the simpler hypothesis that the inorganic 
colloidal material is chiefly the hydrous oxides whose adsorption 
capacity for salts is well known. 

Adsorption of Water.—The capacity of the colloidal content of 
the soil to adsorb and retain water is second only in importance 
to the adsorbing capacity for salts. Indeed, Gile and his 
coworkers have found that 95 per cent of the adsorptive capacity 


TaBLeE XLII 
Hygroscopic Heat evolved? 
Type of soil coefficient, ! by 50 grams of 
per cent soil, calories 

Gg OLN EEE” oS jie: 0.0 
TSS SS SG Be 8 2 0.5 0.2 
WOW Cabal p00 gag 1.5 0.8 
rei Re rg ee ee es Das 10.8 
PPPS MPR LORING ek cd es ee eee es 6.5 15.0 
Fee ee re fe er 2 ieee se ee 9.8 172.8 
pile 1 ta Oo ee ee 205.2 
Lic MOS ELE OO 9 ee a a 11.8 391.5 
SOUS ES 2 8 0) RO 6 SEO Fa 14.6 607.5 


1 Briees and SHantz: U.S. Dept. Agr., Bur. Plant Ind., 230 (1912). 
2 Bouyoucos: Mich. Agr. Exp. Sta., Tech. Bull. 42 (1918); cf. Mtnrz and GaupEcHon: 
Ann. sci. agron., (3) 4, 393 (1919). 


of the soil is due to its colloidal material and only 5 per cent to 
the non-colloidal material. If dry soils are placed in a saturated 
atmosphere, they adsorb water until a condition of approximate 
equilibrium is attained. The amount of adsorbed water and the 
heat of adsorption increase with the fineness of the particles, as 
shown in Table XLII. These observations were made by differ- 
ent investigators and on different soils which come under the 
same general soil type. 

Bouyoucos! has proposed the phenomenon of heat of wetting 
as a means of estimating the colloidal content of the soil. As 
would be expected, the colloids of different soils vary widely in 
their heat of wetting, owing to the difference in their physical 


1 Soil Science, 16, 320 (1924). 


408 THE HYDROUS OXIDES 


character. Heating to 750° is said to decrease the heat of wetting 
of soils to zero. This cannot be strictly true, for the adsorptive 
capacity of ignited soils may be 30 to 50 per cent of the value 
before ignition. Because of this loss of adsorptive capacity on 
ignition, Alway! questions the reliability of the water-adsorption 
method of estimating the colloid content of the soil. This seems 
to be beside the point, since one might reasonably expect the 
coalescence accompanying ignition to decrease materially the 
amount of colloidal matter. The validity of the water-adsorp- 
tion method depends primarily on whether non-colloidal material 
in unheated soil adsorbs an appreciable amount of water under 
the conditions of determination. 

The total water-holding capacity of a soil is influenced to a 
considerable extent by the height of the soil column and by the 
mode of packing of the particles; but the colloidal content is by 
far the most important factor in determining the moisture-hold- 
ing capacity. Bouyoucos? found that some ordinary clays will 
hold as much as 75 per cent of water as compared to only 20 per 
cent in some coarse sands. The best method of increasing the 
colloid content and, hence, the water-holding capacity of sandy 
farming land is to increase the organic matter by the application 
of good farming methods. 

Not only do the colloidal particles adsorb and conserve water 
for times of drouth, but the freezing point of water is lowered 
very appreciably when it is adsorbed.* As in the case of the 
hydrous oxide gels,‘ a part of the adsorbed water is not frozen 
until the temperature is reduced several degrees below zero. 
This is doubtless of importance in preventing complete desicca- 
tion of the soil by freezing and the consequent destruction of the 
soil bacteria. | 

Plasticity.—Certain colloidal material acts as a binder, and if 
present in suitable amount, it holds the particles of soil together 

1 Auway: Colloid Symposium Monograph, 8, 241 (1925); Pur1, CRowTHER, 
and Krrn: J. Agr. Sci., 15, 68 (1925). 

> Colloid Symposium Monograph, 2, 132 (1924); cf. Kina: Wis. Agr. 


Exp. Sta., Sixth Rept., 189 (1889); Auway and Kina: J. Agr. Research, 14, 
27 (1917). 

® Bouroucos and McCoou: Mich. Agr. Exp. Sta., Tech. Bull, 31 (1916); 
36 (1917); Parker: J. Am. Chem. Soc., 48, 1011 (1921). 

4 Foore and Saxton: J. Am. Chem. Soc., 38, 588 (1916). 


THE SOIL 409 


in a granular structure, thus preventing them from being blown 
or washed away and providing for aeration. The quantity of 
colloidal matter in a soil does not differ greatly, as a rule, from 
the quantity of the “clay fraction” given by various systems of 
mechanical analysis. Guile! points out, however, that in certain 
cases the nature of the clay fraction may be a more important 
factor in determining how a soil will act than the quantity of this 
fraction.2 It is possible to increase the colloidal content of 
sandy soil by the direct addition of a plastic clay and to cut down 
the plasticity of a clay soil by the addition of sand; but this method 
of controlling the relative proportion of suitable colloidal to 
non-colloidal material is too expensive to use in the ordinary 
farming operations. Sand-clay roads, however, are constructed 
by mixing sand with plastic clay, which serves as a binder. 
Acidity of the Soil.—The so-called acidity of the soil is prob- 
ably due in large measure to selective adsorption. If one shakes 
fuller’s earth with distilled carbon-dioxide-free water and filters, 
the filtrate is neutral to litmus and to phenolphthalein, showing 
the absence of soluble base or acid. Now, if a dilute sodium 
chloride solution is shaken with fuller’s earth and filtered, the 
filtrate is acid to litmus or the phenolphthalein. Obviously, 
this is not because the fuller’s earth is acid, but because it adsorbs 
the base from the sodium chloride solution more strongly than 
the acid, giving the solution an acid reaction. Similarly, if a 
piece of litmus paper is pressed against moistened fuller’s earth, 
the paper turns red, and if fuller’s earth is added to a faintly 
alkaline solution of phenolphthalein, the red color disappears. 
Bancroft reports that the adsorbing power of fuller’s earth is so 
great that an acre-foot, as soil, would adsorb 30,000 pounds of 
lime, thus making the fuller’s earth about equivalent in acidity 
to a 2 per cent solution of sulfuric acid. Not only do clays and 
certain hydrous oxides, such as hydrous silica and manganese 
dioxide,‘ show this selective adsorption, but van Bemmelen® 


1Cf. Proc. Am. Soc. Civil Eng., 51, 892 (1925). 

2 MippLETON: J. Agr. Research, 28, 499 (1924). 

3Campron: J. Phys. Chem., 14, 400 (1910); Bancrort: “Applied Col- 
loid Chemistry,” 121 (1921). 

4Van BEMMELEN: “Die Absorption,” 445 (1910). 

5 “Die Absorption,” 454 (1910). 


410 THE HYDROUS OXIDES 


reports that colloidal humus substance decomposes small amounts 
of solutions of ammonium chloride, carbonate, phosphates, and 
borates, the base being adsorbed more strongly than the acid, 
giving the solution an acid reaction. The same results are 
obtained by digesting either a humus-rich or a clay-rich soil witha 
solution of ammonium chloride. Gile! showed conclusively that 
silica gel has a beneficial action on the growth of plants supplied 
with rock phosphate by increasing the quantity of phosphoric 
acid in solution. This is due to decomposition of the rock phos- 
phate by stronger adsorption of hydroxyl ion than of hydrogen 
ion by the silica gel. 

In the light of these observations, it appears evident that a 
part and possibly the larger part of the so-called soil acidity 
results from selective adsorption of the basic constituent of 
certain salts.2, This view has been supported by Salter and.Mor- 
gan® as a result of recent observations on the change in acidity 
of certain soils with variation in the soil-water ratio. In general, 
it was found that the variation in hydrogen ion concentration 
agrees with the distribution of hydrogen ions between soil and 
solution which could be expected if controlled by an adsorption 
mechanism. The conclusion is reached that the reaction of a 
soil is dependent on three factors: the total dissociated acid pres- 
ent; the adsorptive capacity of the soil for hydrogen ion; and the 
soil-water ratio. 

The selective adsorption theory of soil acidity is opposed by 
those who believe that the acidity is due to aluminosilicic acids 
and humic acid which are relatively insoluble but are soluble 
enough to give the soil solution an acid reaction. Bradfield 
gets a kind of end point on titrating dilute solutions of strong 
bases with acid colloidal clays by either the conductivity or 
hydrogen electrode method. This is considered as proof of a 
neutralization reaction between a strong base and weak soil 
acid, the anion of which is a particle of colloidal dimensions. 


1 Gite and Smitu: J. Agr. Research, 31, 247 (1925). 

2 Harris: J. Phys. Chem., 18, 335 (1914); Noyss: J. Ind. Eng. Chem., 
11, 1040 (1919); Kappmn: Landw. Vers.-Sta., 96, 306 (1920); Matrson: 
Kolloidchem. Bethefte, 14, 296 (1922). 

oJ. Phys. Chem: 27, 117 (1928), 

4J. Am. Chem. Soc., 45, 2669 (1923). 


THE SOIL 411 


Returning to the case of fuller’s earth and salt referred to at the 
beginning of this section, one may write the equation for the 
hydrolysis of sodium chloride as follows: 


Na + Cl’ + H.0 5 Na’ + OH’ + H’ + Cl’ 


Since fuller’s earth adsorbs hydrxhyl ion more strongly than 
hydrogen ion, it displaces this equilibrium to the right, giving 
the solution an acid reaction. Now if one adds a base, it will 
tend to displace the equilibrium in the opposite direction, and 
one will obtain what amounts to an end point when the amount of 
alkali added just neutralizes the increased tendency of sodium 
chloride to hydrolyze as a result of preferential adsorption of 
hydroxyl ion by the fuller’s earth. But this is an entirely differ- 
ent thing from fuller’s earth itself being a weak acid that is 
neutralized by a strong base. Stating the matter in another 
way: In the presence of a certain concentration of hydroxy] ion,. 
the adsorption capacity of the fuller’s earth is satisfied for this 
ion, and the hydrolysis of the sodium chloride remains the same 
as in the absence of fuller’s earth. ‘The concentration of all 
‘strong bases required to bring about this result would be the 
same, provided the cations of all bases are adsorbed equally and 
have the same effect on the adsorption of hydroxyl ion. Actu- 
ally, the cations of strong bases are not all adsorbed to the same 
extent, and the concentration will not be identical for different 
bases. For the present at least, there appears no reason for 
regarding the so-called end point in the titration of acid soils as 
proof of the existence of a definite soil acid which yields a colloidal 
anion. 

It should be mentioned in passing, that pseudo end points are 
not infrequently encountered in adsorption phenomena. ‘Thus, 
in the exchange adsorption with strongly polar adsorbents! 
such as kaolin, one gets what might be termed end points at 
quite similar concentrations of different salts of the same cation. 


Loe 
Moreover, when the value 7 the Freundlich adsorption 


formula? is small, the adsorption curve may bend relatively 
_ sharply and take a direction nearly parallel to the concentration 


1 FREUNDLICH: “Kapillarchemie,”’ 279 (1922). 
2 FREUNDLICH: “ Kapillarchemie,” 156 (1922). 


412 “THE HYDROUS OXIDES 


axis, thereby giving what might be interpreted as an end point 
above which the adsorption increases but little with increasing 
concentration. 

Bradfield! determined the hydrogen ion concentration of vary- 
ing concentrations of colloidal clay and compared the results 


Normality of Acetic Acid 
0.02 004 0.06 008 010 Ole 014 











46 | i 

5 ee 
pianos 
- 


Ei eee of fen Ch vote 


Fig. 31.—The effect of concentration of colloidal clay and acetic acid upon the 
hydrogen ion concentration. 


with similar determinations on acetic acid. Colloidal material 
was extracted from an acid clay by the aid of the supercentrifuge, 
and a sol was prepared containing 12.8 per cent of oven-dried 
material. From this stock solution, dilutions containing 6.4, 
3.2, 1.2, 0.8, 0.4, 0.2, 0.05, and 0.25 per cent were prepared and the 
hydrogen ion concentration determined. Various concentrations 


1J, Phys. Chem., 28, 170 (1924). 


THE SOIL 413 


of acetic acid from 0.000025 to 0.1 N were also prepared and 
their hydrogen ion concentrations determined. ‘The results are 
plotted in Fig. 31. It will be seen that the relationship between 
concentration of acetic acid solution and its hydrogen ion con- 
centration is nearly linear at very low concentrations and 
becomes exponential at higher concentrations. Similarly, the 
relationship for colloidal clay is about linear at high dilutions, 
exponential at intermediate dilutions, and almost constant at 
higher concentrations. The similarity in the two curves leads 
Bradfield to regard the colloidal clay as a weak acid which behaves 
like acetic acid. Personally, I cannot see how the evidence 
justifies this conclusion. It would seem that with equal pro- 
priety one might assume that the acid or mixture of acids formed 
as a result of preferential adsorption of hydroxyl ion behaves 
similarly to acetic acid as regards change in pH with increasing 
concentration. + 

Truog? likewise supports the view that the acidity of soils is 
due to the presence of relatively insoluble aluminosilicic acids, 
but claims that the electrometric method is unsuitable for deter- 
mining the hydrogen ion concentrations of the very slightly 
buffered solutions such as are obtained with relatively low soil- 
water ratios. ‘The reason for this is that slight diffusion of 
potassium chloride from the connecting bridge, slight contamina- 
tion of alkali from glassware, slight impurities in the hydrogen, 
presence of nitrates in the soil, and shghtly contaminated or 
so-called poisoned electrodes can easily effect the reaction of 
slightly buffered soil suspensions and solutions.” He, therefore, 
determined the hydrogen ion concentration of soil-water extracts 
colorimetrically after filtering out all the colliodal matter with a 
special ultrafilter. When the soils were thoroughly washed to 
remove excess soluble salts, the hydrogen ion concentrations of 
the ultrafiltrates appeared to be fairly constant at soil-water 
ratios of 1 to 2, 1 to 20, and 1to50. Salter and Morgan failed to 
obtain a constant hydrogen ion concentration measured poten- 
tiometrically at varying salt-water ratios, and so concluded that 
soil acidity was not due to complex soil acids. The constancy 
in hydrogen ion concentration at varying soil-water ratios as 


1Wawpo.e: J. Chem. Soc., 105, 2521 (1914). 
2 Colloid Symposium Monograph, 3, 228 (1925). 


414 THE HYDROUS OXIDES 


determined by Truog’s method was offered as proof that acidity 
of the soil is due to colloidal acids. 

Now if the acidity is due to relatively insoluble colloidal acids, 
the surface ionization will give hydrogen ion and a cation of 
colloidal dimensions, as claimed by Bradfield. When such a 
suspension is filtered through an ultrafilter which holds back all 
the negatively charged colloidal particles, it will obviously hold 
back their hydrogen ion equivalent, so that from this point of 
view, the hydrogen ion concentration determined as Truog does it 
is due entirely to the molecularly dissolved complex acid. Since 
the degree of dissociation of such an acid is probably very slight 
even at high dilutions, the solubility of “clay acid” necessary to 
get a pH value of 4 would be quite appreciable. Truog should 
make a careful investigation of his perfectly clear ultrafiltrates; 
for if he can show that these ultrafiltrates contain only complex 
aluminosilicic acid and humic acids in molecular solution, then 
the problem is solved. It is altogether unlikely that the alleged 
complex acids, if they exist, are as soluble as Truog’s data would 
suggest. Until we know more of the nature and composition of 
Truog’s ultrafiltrates, it 1s impossible to give an intelligent 
interpretation of his observations. . 

It should be mentioned, in conclusion, that Schreiner and 
Shorey,! Olin,? and others have demonstrated the existence in 
the soil of definite compounds possessing an acid character; — 
but the cases in which these compounds are present in sufficient 
quantities to give an acid reaction are rare. 


FLOCCULATION AND DEFLOCCULATION 


A suspension of soil colloid is made up of negatively charged 
particles and is, therefore, flocculated readily by the addition of 
salts containing cations that are relatively strongly adsorbed. 
As a result of his investigations on the flocculation of kaolin, 
Bodlander? introduced the term ‘‘threshold value” of electrolytes, 
which was defined as the concentration necessary to cause rapid 
flocculation. Hall and Mouson‘* determined the precipitation 

1 U. 8. Bur. Soils, Bull. 47, 70, 74, 77, 80, 83, 87, 88, 98. 

2 Ber., 45, 651 (1912). 

3 Jahrb. Mineral., 2, 141 (1893). 

4 J. Agr, Sct., 2, 251 (1907). 


THE SOIL 415 


concentration of various chlorides, sulfates, and nitrates on 
colloidal clay. The order of precipitating power of the cations 
beginning at the greatest is: hydrogen, aluminum > calcium, 
barium, magnesium > potassium > sodium; and the order of 
stabilizing power of the anions is hydroxyl > sulfate > nitrate > 
chloride. ‘The order of a series of acids beginning with hydro- 
chloric, which has the greatest precipitating power, is: hydro- 
chloric > nitric > sulfuric > mono-, di-, and tri-chloracetic 
> acetic > oxalic, tartaric > amido acetic, citric, phenol. 
The last three exert no precipitating action. 

Since colloidal clays owe their charge to preferential adsorption 
of hydroxyl ion, one should expect the precipitation value of 
hydroxides to be higher than that of neutral salts. Bradfield! 
reports that 1.4 milliequivalents of potassium are required to 
coagulate a certain soil colloid when present as chloride, and 14 
milliequivalents as hydroxide; while 10 milliequivalents are 
required with a mixture of 19 parts chloride and 1 part hydroxide; 
and 14 milliequivalents, with a mixture of 9 parts chloride and 1 
part hydroxide. 

The precipitation value of an electrolyte for a sol is that con- 
centration which results in sufficient adsorption of the precipitat- 
ing ion to neutralize the combined adsorption of the original 
stabilizing ion and the stabilizing ion added with the precipitating 
electrolyte or mixture of electrolytes.2 The precipitation value 
of potassium chloride is much lower than of potassium hydroxide, 
since chloride ion is adsorbed much less strongly than hydroxyl 
ion by colloidal clay. Mixtures of potassium chloride and 
hydroxide cause coagulation at some value in between the values 
for the individual electrolytes. Obviously, the effect of hydroxyl 
ion will be much greater at relatively low concentrations on 
account of the relatively greater adsorption; and above the 
saturation value for the adsorption of hydroxyl ion which is 
reached fairly sharply in the case of a strong adsorbent for a 
strongly adsorbed ion such as clay for hydroxyl, the precipitation 
value of potassium ion is fairly constant. Another factor which 
may come in is that, above the normal saturation value, the 
presence of the strongly adsorbed hydroxyl ion may actually 


1J. Am. Chem. Soc., 45, 1243 (1923). 
2 WeIsEeR: J. Phys. Chem., 25, 680 (1921). 


416 THE HY DROUS OXIDES 


increase the adsorption of the precipitating cation to such a 
degree that the rate of precipitation in the presence of the mixture 
is greater than that of the same concentration of salt without any 
added hydroxide. This is apparently what happens in certain 
cases as observed by Mattson! in Ehrenberg’s laboratory. Matt- 
son finds the order of precipitating power of calcium compounds 
for a negatively charged colloidal clay to be: calcium chloride > 
calcium sulfate > calcium bicarbonate > calcium hydroxide; 
but when a concentration a little above the precipitation value of 
calcium hydroxide is attained, the rate of flocculation is faster 
than is observed for salt concentrations considerably above their 
respective precipitation values. Similarly, when small amounts 
of sodium hydroxide are added to the clay sol, the stability is 
increased, as evidenced by the higher concentration of calcium 
sulfate required for flocculation. But when the initial concen- 
tration is increased to 0.002 N, in a 1 per cent clay, the rate of 
precipitation is appreciably greater than with calcium sulfate 
alone, even though the concentration of the latter is considerably 
above its precipitation value in the absence of sodium hydroxide. 
It appears obvious that, above a certain concentration, the 
influence of hydroxyl ion in increasing the adsorption of the 
precipitating calcium ion predominates over its own stabilizing 
action. Mattson showed that the presence of hydroxyl ion 
increases enormously the adsorption of calcium ion by quartz. 
Comber? attributes the abnormal flocculating power of calcium 
hydroxide above a certain concentration to its coagulating action 
on emulsoid matter that tends to stabilize the clay sol. 

Alkali hydroxides in low concentration have a stabilizing action 
on colloidal clay, while higher concentrations cause flocculation. 
In the ceramic industry, the so-called clay slip is prepared by 
deflocculating clay with sodium hydroxide, carbonate, or silicate. 
The slip can be readily poured or cast, even though it contains 
less water than a stiff mass of clay and water without alkali. 
Adding a little acid to a fluidified clay slip flocculates the mass 
which becomes so stiff that it will not fall from an inverted vessel. 
Clays carrying appreciable amounts of soluble salts, such as 

1 Kolloidchem. Beihefte, 14, 241 et seq. (1922); cf. Compmr: J. Agr. Scz., 11, 


450 (1922); Fopor and ScHoENFELD: Kolloidchem. Bethefte, 19, 1 (1924). 
2 J. Agr. Research, 12, 372 (1922). 


THE SOIL 417 


the sulfates of calcium and magnesium, are difficult to defloecu- 
late; while clays containing protective colloids, such as humus, 
are readily peptized. 

The deflocculating action of calcium hydroxide is not as marked 
as that of the alkali hydroxides, because of the relatively strong 
precipitating power of calcium ion. Nevertheless, it appears 
that calcium hydroxide in low concentrations may have an appre- 
ciable stabilizing influence on colloidal clay. The addition of 
lime to soil containing a large amount of deflocculated colloidal 
material is intended to impart a crumbly flocculent structure to 
the soil; but in certain instances, liming is reported to have an 


TasBLE XLIITIL—FLoccuLaTIon AND DEFLOCCULATION OF CLAY BY LIME 


Concentration in milliequivalents per liter of 
Character of superna- 


Ca(HCO;). Ca(OH), tant solution 
2.02 ey Clear 
2.02 1.35 Slightly cloudy 
2.02 Le5r : Cloudy 
2.02 1.80 Very cloudy 
2.02 2.02 Very cloudy 
2.02 2,25 Cloudy 
2.02 2.47 Slightly cloudy 
oe 2.70 Clear 
2.02 2.92 Clear 
2.02 Boho Clear 


unfavorable effect on the structure. Mattson! flocculated a 
colloidal clay with calcium bicarbonate and then treated it with 
varying concentrations of lime water, with the results recorded in 
Table XLIII. It will be seen that calcium hydroxide in certain 
concentrations does have a peptizing action on clay containing 
bicarbonate, and it is probable that a similar condition may be 
encountered in a clayey soil if the lime has been used too spar- 
ingly. When lime is added to the soil, it is converted into the 
hydroxide, a part of which is adsorbed and another part of which 
is neutralized by the bicarbonate present. If the amount of lime 
added is sufficient to neutralize the deflocculating bicarbonate 


1 Kolloidchem. Beihefte, 14, 276 (1922). 


418 THE HYDROUS OXIDES 


and not enough to neutralize the adsorbed hydroxy] ion, then the 
lime will have an unfavorable influence on the soil structure. 

Under certain conditions, sodium salts! in the soil are converted 
in part into soda which has a strong deflocculating action on the 
colloidal material. If'the soil in question is permeable or sandy, 
the colloidal hydrous oxides and humus are washed down by the 
rain to a lower stratum, the depth of which is determined by 
the rainfall in the locality. There, the collected mass of colloidal 
material and fine sand hardens by desiccation forming an insolu- 
ble rock-like layer known as hardpan. This formation may shut 
off the soil beneath from air and water and, by interfering with 
drainage, may bring about swampy conditions. The addition 
of a suitable amount of gypsum to a soil containing soda, neutral- 
izes the deflocculating action of the latter, owing to strong adsorp- 
tion of calcium ion. 


1 HinGarD: ‘‘Soils,’’? 62 (1906); Enrensmera: ‘Die Bodenkolloide,’’ 347 
et seq. (1922). 


AUTHOR INDEX 


Abegg, 1382, 202 
Aboulene, 154 
Adams, 308, 359, 361 
Adler, 316 

Adolf, 116, 242, 243 
Ahrndts, 169-171 
Alexander, 7, 10, 12 
Allen, A. H., 207 
Allen, E. T., 104, 107, 118, 173, 291 
Allmand, 174 
Aloy, 292, 293 
Alvarez, 308 

Alway, 408 
Amberger, 172, 310 
Andersen, 71 
Anderson, J. 8., 7, 177, 178 
Anderson, M. 8., 397, 401, 402 
‘Anderson, W. C., 165 
Anschiitz, 250 
Antony, 72, 208, 219 
Apostolo, 334 
Appleby, 228 
Appleyard, 286, 339 
Archibald, 119 

Arisz, 17 

Arnold, 27 

Aron, 217 

Arppe, 277 
Arrhenius, 10 
Asbury, 155 

Ashley, 397 

Aspdin, 383 

Atkin, 325 

Atterberg, 160 
Auerbach, 226, 230 
Auger, 234 
Austerweil, 229 
Austin, 294 
Avogadro, 85 


Bach, 57, 301 

Bachmann, 7, 8, 10, 11, 16, 23, 178, 
194, 273 

Baenziger, 353 

Baerwind, 310 

Bahr, 247 

Baikov, 391 

von Baikow, 76 

Bailey, 245 

Bailhache, 284 

Balderston, 323 

Baldwin, 324, 328, 329 

Balz, 154, 291 

Bancroft, E., 336, 345, 346 

Bancroft, W. D., 6, 18, 25, 32, 74, 
89, 100; 112,. 136," 137, 287; 
340, 342, 346-348, 357, 358, 
365, 409 

Banerji, 371, 375 

Barab, 43, 82, 83, 88 

Barbera, 301 

Barclay, 188 

Barfoed, 203, 206, 207, 217, 218 

Barnes, J., 235, 245, 338 

Barnes, 8. K., 162 

Barr, 114 

Barratt, 9, 11 

Bartell, 16, 189 

Bartels, 153 

Bartman, 301 

Baskerville, 254, 260 

Bassett, 86 

Basu, 143 

Bates, 390, 394 

Bayer, 107 

Baylis, 374 

Bayliss, 128, 245, 364 

Bechhold, 57, 370 

Becker, 77, 81 


419 


420 


Becquerel, 104, 108, 135 

Bedford, 153 

Beech, 344, 348, 349 

Behr, 178, 184 

Beilby, 72 

Belden, 238 

Bellucci, 155, 203, 209, 230, 231, 309, 
312, 313 


van Bemmelen, 1, 6, 7, 22, 23, 3C, 


35-37, 42, 76, 81, 104, 106, 107, 
134, 135, 137, 160, 164, 165, 
175, 177, 178, 200, 204, 206, 
207, 209, 211, 238, 239, 248, 
2738, 275, 290, 294, 295, 396 

Benedicks, 262 

Benedict, 148 

Bennett, 325 

Benson, 393 

Bentley, 115 

Berger, 154 

Berl, 229 

Bertheim, 281 

Berthier, 303 

Berthold, 67 

Bertrand, 301, 303 

Berzelius, 79, 81, 157, 164, 202, 203, 
219, 244, 282-284, 315 

Betz, 147 

Bied, 394 

Biedermann, 23 

Biehler, 311 

Billitzer, 64 

Billy, 237 

Biltz, M., 49, 86 

Biltz, W., 42, 62, 67, 89, 113, 216, 
241, 246, 254, 264, 278, 285, 
289, 292, 331, 358, 359, 361 

Bird, 174 

Birnbaum, 133 

Biron, 207, 217 

Bishop, 57, 59, 60 

Bissel, 262 

Bjerrum, 83-86, 89, 91, 118 

Black, 90 

Blanck, 301 

Blank, 200, 201 

Blencke, 129 


THE HYDROUS OXIDES 


Bleriot, 240 

Bleyer, 161, 163, 358 

Blondel, 312 

Bloxsom, 28 

Blucher, 137, 365 

Blum, L., 118 

Blum, W., 116, 117, 119 

Blumenthal, 389 

Bobertag, 9 

Bodlander, 370, 414 

Boedecker, 169 

Boehringer & Sons, 346 

Boelter, 111 

Bogaers, 190 

Bogert, 370 

Bogojawlenski, 365 

Bogue, 9, 11, 16, 404 

Bohm, 38, 103, 109, 114, 159, 163, 
164, 169, 238, 240, 247, 258, 
254, 260-262 

Boisbaudran, Lecogq de, 129, 250, 254 

Boltzmann, 5 

Bonnet, A., 229 

Bonnet, F., 86 

Bonsdorff, P. A., 107 

Bonsdorff, W., 135, 152 

Bontemps, 146 

Borcherdt, 199 

Borjeson, 15 

Bornemann, 309 

Boswell, 68 

Béttger, 135, 137, 226 

Boéttinger, 339 

Bourion, 78 

Bouyoucos, 401, 407, 408 

Bowers, 171 

Boyer, 131 

Bradfield, 48, 48, 57, 68, 73, 125, 148, 
194, 396, 398, 400, 410-415 

Bradford, 9, 11, 169 

Braesner, 259 

Brauner, 253 

Bredig, 45, 296 

Brescius, 34 

Bridgeman, 129 

Briggs, H., 192 

Briggs, L. J., 407, 


AUTHOR INDEX 


Briggs, T. L., 392 

Britton, 162 

Brizard, 305 

Brizzi, 271 

Brossa, 64 

Brown, 192 

Browne, F. L., 43, 46, 52, 57, 67 
Browne, R. J., 317 

Bruce, 156 

Bruni; 198, 281 

Briinjes, 154, 283 

Bruyn, Lobry de, 147, 303 
Bryliuski, 245 

Buchner, 9, 23, 32, 173 
Bull, 359, 361 

Bunce, 100, 174 

Bunson, 67, 76 

_ Binz, 278 

Burger, 280, 288, 289 

_ Burgstaller, 152 

Burrell, 190 

Burton, D., 17 

Burton, E. F., 57, 59, 60, 142, 227 
Bury, 224, 225 

Buswell, 374, 376 
Butschli, 5, 7, 8, 23 
Bittner, 283 


Caleagni, 230 

Cameron, 47, 409 

Campbell, 165, 167, 387, 388, 394 

Carnegie, 133 

Carnelley, 104, 131, 133, 155, 173, 
204, 233, 253 

Carnot, 151 

Carobbi, 260 

Caron, 111 . 

Carrara, 118 

Carson, 225 

Cassius, 218 

Casthelez, 80 

Castro, 310 

Catlett, 260, 375 

Chance, 69 

Chaney, 188, 191 

Charriou, 126, 379 

Chase, 258 


421 


Chattaway, 145 

Chatterji, 44, 89, 116, 148, 148, 299 

Chaudrion, 291 

Chen, 271 

Chiari, 17 

Chilesotti, 284 

Chrétien, 48 

Christenson, 304 

Church, 89, 222 

Clark, F. W., 276, 277 

Ciat Creel oD 

Clark, W. M., 368, 371, 373, 374, 
377, 380 

Classen, 208, 237 

Claus, C., 305-307, 309, 311 

Claus, C. F., 69 

Clavari, 155 

Cleve, 245-247, 250, 254, 260-262 

Coehn, 152 

Coggeshall, 30 

Cohen, 174 

Collins, 207, 210, 214, 217, 218 

Comber, 416 

Comey, 171 

Condrea, 71 

Conrad, 274 

Cooke, 150, 154 

Coolidge, 184, 188 

Corfield, 278 

Cossa, 104, 108 

Cotton, 54, 55 

Cottrell, 171 

Coxe, 377 

Creighton, 148, 144 

Crenshaw, 173 

Croll, 201 

Crombie, 43 

Crooks, 259 

Crow, 271 

Crowther, 408 

Crum, 104, 112, 338 

Cushman, 30 


Dallyn, 375 
Damiens, 258, 259 
Dammer, 70 
Daniels, 191 


422 


D’ Ans, 69 

Darke, 9 

Daubrawa, 274 

Davidheiser, 183 

Davidson, 280 

Davies, 34 

Davis, E. C. H., 181 

Davis, G. H. B., 75 

Davis, R. O. E., 397 

Davison, 325, 329, 358 

Davy, 202 

De Boeck, 295, 299 

Debray, 41, 47, 219, 306, 307, 310 

De Forcrand, 140, 169, 170 

Deichler, 278 

Deisz, 11, 29, 297 

Delacroix, 274, 276 

Delaporte, 375 

Demoly, 238, 234 

Denham, 83 

Dennis, 129, 181, 199, 200, 258, 323 

Desch, 47, 114, 248 

Deville, 111, 164, 166, 310 

Dhar, 44, 59, 89, 98, 116, 142, 148, 
148, 150, 151, 168, 278, 299 

Dickson, 68 

Diesselhorst, 266, 269, 291 

Ditte, 107, 164, 165, 224, 226, 264 

Dollifies, 198 

Donath, 148, 150 

Donnan, 17-19, 21, 85, 144, 318, 
319 

Douglas, 332 

Dozzie, 45 

Dreaper, 355, 365 

Dreyer, 332 

Drummond, 240, 249 

Duchez, 390, 391 

Duclaux, 47, 49, 53-56 

Dudley, 157, 312 

Dullberg, 263, 265 

Dullo, 108 

Dumanski, 47, 53, 54, 266, 285 ° 

Dumas, 315 

Durfee, 348 

Dutailly, 146 

Dutoit, 171 


THE HYDROUS OXIDES 


Eberman, 184, 187 
Kbler, 145, 194 
Ebner, 77 


Eckardt, 225 


Edison, 153 

Edwards, 271, 374 
Ehrenberg, 397, 399, 418 
Ehrhart, 310 

Ehrlich, 289 

Hitner, 254 

Elbs, 135 

Elliot, 151 

Elliott, 8 

Endell, 79, 394 

Engel, 203, 205-207, 214 
Engels, 281, 289, 290 
Englehardt, 146 

Engler, 237 

Erdmann, 153, 154 
Erikson, 128 

Erk, 253 

Errera, 268 

Espel, 154 

Kuler, H., 128, 129, 137, 260 
Euler, U., 137 


Fahrion, 321 

Failyer, 406 

Fall, 27 

Faraday, 72 

Farnau, A., 254, 256, 258 

Farnau, E. F., 187, 149, 151, 254, 
365 

Fehrmann, 230 

Fellner, 194 

Fells, 192 

Férée, 76 

Feucht, 150 

Field, 8 

Fieldner, 190 

Figuier, 157 

Finch, 100, 144 

Fink, 392 

Firth, 192 

Fischer, A., 7 

Fischer, H. W., 35, 45, 47, 67, 68, 71 
83, 89, 148 ; 


AUTHOR INDEX 


Fischer, M., 17 

Fischer, W., 9, 89, 101, 229 

Flade, 9 

Flemming, 197 

Flenner, 403, 405 

Flinn, 370 

Fodor, 399, 416 

Foerster, 144, 155, 171 

Follenium, 172 

Foote, 37, 408 

Foster, 322, 328 

Fouts, 254 

Fowles, 137, 138 

Franke, 303 

Frankenheim, 8 

Franz, 210 

Fremery, 205 

Fremy, 76, 81, 111, 145, 147, 203, 
204,. 207, 209, 217, 274, 294 

Fresenius, 118 

Freundlich, 6, 42, 48, 55, 62-66, 70, 
94, 95, 120-122, 128, 148, 159, 
168, 184, 264-267, 283, 286, 291, 
310, 321, 331, 332, 411 

Frey, A., 248 

Frey, W., 312 

Fricke, 77, 78, 103, 109, 110, 129, 
130, 169-171 

Friedemann, 57, 60 

Frieden, 47, 48, 317 

Friedrich, 119 

Friend, 75 

Fritzsche, 263 

Fry, 397 

Fuller, 162 

Fulton, 191 

Furness, 191 

Firstenhagen, 339: 


Gadd, 392 

Gain, 271 

Galecki, 63 

Gallun, 320, 328 

Ganguly, 299 

Gann, 120 

Ganswindt, 344, 346, 347, 350, 355 
Gardner, 151, 348 


423 


Garelli, 254, 334 

Garnier, 261 

Gary, 394 

Gassner, 393, 394 

Gaudechon, 407 

Gaurilow, 438 

Gay Lussac, 112, 173, 202, 219 

Geer, 131 

Geloso, 301 

von Georgievics, 286, 339 

Gerland, 270 

Germann, 71 

Germs, 227 

Gessner, 267 

Geuther, 76, 227, 230, 274 

Ghosh, 59, 98, 299 

Gibbs, 287 

Giesy, 47 

Giglio, 72 

Gilbert, 189, 363, 364 

Gile, 397, 398, 400, 401, 407, 409, 410 

Giolitti, 39, 48, 47 

Girard, 208 

Gjaldbaek, 164 

Glasenapp, 391 

Glaser, F., 154 

Glaser, M., 152, 295 

Glasstone, 226-228 

Glazebrook, 174 

Glendenning, 145 

Gmelin, 161 

Gnehm, 353 

Goldmann, 271, 321 

Goldschmidt, 216, 225 

Gonnerman, 195 

Gooch, 294 

Goodwin, 72, 88 

Gordon, D., 70 

Gordon, N. E., 361, 363, 399, 403-405 

Gorgeu, 294, 295, 303 

Goris, 190 

Gortner, 9 

Gottschalk, 291 

Goudriaan, 118, 169-171 

Graham, 28, 33, 42, 82, 118, 141, 193, 
204, 206, 214, 215, 234, 280, 
289, 333 


424 


Green, 191 

Greenfield, 376 

Greider, 181, 184 
Grimaux, 27, 42, 44, 64, 67, 194 
Grimm, 184 

Grinberg, 237 

Grindley, 10 

Grobet, 118, 171 

Groéger, 145, 147 
Groschuff, 194 

Gross, 179 

Grouvelle, 164 

Grover, 88 

Grube, 150 

Griin, 394 

Grundemann, 194 
Gueritte, 395 

Guibourt, 67 

Guichard, 106, 184, 283, 284 
Guignet, 76 

Gunz, 276 

Gustafson, 331 

Gutbier, 146, 278, 305-307 
Guttman, 394 

Guy, 299 

Guyard, 271 

Guzzmann, 88 

Gye, 195 


Habasian, 119 

Haber, 103, 159, 161, 237, 238 

Habermann, 135 

Haeffley, 209 

Hagen, 145 

Hahn, 108 

Hakamori, 120 

Hake, 167 

Hall, A. D., 414 

Hall, V. J., 170 

Hammel, 235 : 

Hance, 200 

Hanes, 152 

Hantzsch, 47, 89, 114, 117, 147-149, 
161, 170, 171, 201, 225, 229, 248 

Hardin, 156 

Hardy, F., 194 

Hardy, W. B., 6-8 


THE HYDROUS OXIDES 


Hargreaves, 69 

Harms, 137 

Harned, 328 

Harris, 405, 410 

Harrison, 11—14 

Hartmann, 172 

Hartung, 23 

Hase, 265 

Hatfield, 374, 375, 377 

Hatschek, 5, 11, 180 

von Hauer, 263 

Hauser, 244, 272, 278 

Haushofer, 200 

Havrez, 339, 342 

Hawley, 229 

Hay, 89, 204 

Hedvall, 110, 136, 148, 149-153, 
165, 233 

Heermann, 353 

Hefftner, 274 

Heinz, 210 

Hermann, 245, 273, 303, 344, 345 

Hertzmann, 243 

Herz, 89, 117, 170, 229, 302 

Herzfeld, 210, 344 

Herzog, 316 

Hess, 265 

Hewis, 277 

Hey, 322 

Heyer, 193 

Heyrovsky, 118, 377 

Hildebrand, 89, 117, 171 

Hilgard, 397, 418 

Hillebrand, 178 

Hober, 70 

Hofmann, 310 

Hofmeister, 17, 146 

Holdcroft, 110 

Holleman, 190 

Holmes, 27, 168, 180, 181, 192, 197, 
398, 401, 402 

Hooker, 143 

Hoover, 373 

Hopkins, 379 

Hoskinson, 284 

Hough, 333 

How, 333 


AUTHOR INDEX 


Howell, O. R., 155 

Howell, W. H., 9 

Hue, 77 

Hulett, 174 

Hummel, 347-350 

Hiimmelchen, 295 

Humphrey, 267 

Hiittig, 30, 31, 37, 280, 288, 289, 293 
Hittner, 151 


Ichlopine, 154 
Ipatiev, 153 
Ishazaka, 120, 126 
Iszard, 200, 201 
Ives, 249 
Iwanitzkaja, 120 


Jackson, 171, 333 
Jacobi, 309 

Jaeger, 227 

Jager, 259 

James, 164, 262 
Jander, 118, 120, 273, 275 
Jandraschitsch, 116 
Javillier, 301 

Jettmar, 333 

Joannis, 140 

de Joannis, C. L., 172 
Jobling, 69 

_ Johnson, E. B., 199 
Johnson, F. M. G., 183 
Johnson, L., 63 
Johnston, 118 

Joly, 305-307, 311 
Jonas, 209 

Jones, D. C., 184 
Jones, G. W., 190 
Jones, H. C., 259 
Jordis, 7, 47, 138, 178, 193 
Jorgenson, 208, 218 
Joye, 261 
Junius, 284 
Justin-Mueller, 321 
Juznitzky, 68 


Kaestle, 250 
Kahle, 195, 196 


Kahlenberg, 144 

Kaiser, 69 

Kaiun, 226 

Kalisch, 195 

Kalle and Co., 174, 3038 

Kappen, 295, 410 

Kappf, 348 

Karrer, 249 

Kast, 201 

Kastner,47 

Kastovski, 63 

Katz, 3, 4, 31 

Kaufmann, S. W., 161, 169, 171 

Kaufmann, W. E., 180 

Kawamura, 120 

Kayser, 301 

Keane, 71, 73, 222 

Keen, 408 

Keisermannn, 166, 389 

Keitschera, 345 

Keller, 45 

Kelley, W., 192 

Kelly, M. W., 320, 322, 324, 329, 330 

Kenngott, 111 

Keppeler, 69 

Kershaw, 355 

Kimura, 156, 171 

King, 408 

Kingsbury, 249 

Klason, 145, 282, 283, 284 

Kleeberg, 163 

Kleeman, 199 

Klein, A. A., 388, 390 

Klein, O., 170, 171 

Klimenko, 259 

Klobbie, 36 

Klobulkoff, 173 

Klosky, 235 

Knapp, 166, 315, 317, 323 

Knecht, 144, 211, 344, 346-351, 
355, 357 

Knop, 235 

von Knorre, 254, 294 

Koechlin, 351 

Koehler, 250 

Koelichen, 225 

Koelsch, 181, 132 


426 THE HYDROUS OXIDES 


Kohlrausch, 50, 180 


Kohlschiitter, 105, 109, 115, 118, 
135, 186, 140-142, 216, 228, 248 


K6nig, 308, 309 
Kopaczewski, 51 
Koppel, 271, 285 
Kowalwsky, 206 
Kramer, 195 
Kraner, 167 
Krantz, 192 
Krauss, 306 

Kraut, 103, 205 
Krecke, 41, 42 
Kremann, 89, 171 
Kreps, 90 

Krieger, 167 

Kroch, 191 

Kroéger, 193, 290, 292 
Kriiger, 138 

Kriss, 157, 161, 163 
Kruyt, 57, 60, 61, 255, 258, 268 
Kithl, Hans, 392-395 
Kuhl, Hugo, 203 
Kuhn, A., 278 
Kiihn, A., 195 
Kiihnl, 47 
Kikenthal, 306 
Kunitz, 21 
Kunschert, 170, 171 
Kuriloff, 170, 172 
Kurre, 280, 288 
Kiister, 144 
Kuznitzky, 45 
Kyropoulos, 179 


Lafuma, 393 
Laing, 9, 10 

Lamb, 184 

Lamy, 133 

Lander, 396 
Langmuir, 189, 267 
Larsen, 227 
Latshaw, 191 

Lea, 156, 277 
Lebeau, 164, 293 


Le Chatelier, 18, 79, 110, 165, 179; 


384, 389 


Lefort, 34, 75, 76 

Lehman, 296 

Leick, 180 

Leide, 179, 194, 311 

Leighton, 347, 355 

Leiser, 292 

Lenher, 15, 157, 178, 195 

Lenker, 201 

Leonard, 194 

Leonhardt, 264, 266, 269, 283 

Lepez, 220, 221 

Leuchs, 306 

Leune, 80 

Leuze, 146 

Levache, 303 

Levi, 154 

Levy, 237 

Lewis, 394 

Lewite, 272 

Ley, 113, 142, 161 

Leyte, 117 

Lichtenwalner, 403, 405 

Liebermann, 337 

Liebig, 311 

Liechti, 116, 339, 348, 345, 347-350, 
358, 359, 361 

van Liempt, 289, 290, 291 

Liesegang, 181 

Linck, 390, 391 

Lindenbaum, 270 

Linder, 47, 54, 94, 169 

Linebarger, 281 

Lipmann, 56 

Lloyd, 7 

Locke, 271 

Lockemann, 68, 357 

Loeb, 17, 21, 22 

Loewel, 76, 81, 83 

Long, 276, 277 

Lonnes, 237 

Lorenz, 135, 145, 170, 203, 205, 207, 
216, 217, 226 

Lorn, 183 

Losana, 139 

Lésenbeck, 193 

Losev, 286, 331 


AUTHOR INDEX 


Lottermoser, 51, 52, 115, 119, 142, 
146, 156, 220, 234, 281, 290, 
291, 296, 297, 332 

Louth, 146 

Lovelace, 189 

Léw, 148 

Lowe, 104, 277 

Léwenstein, 178 

Lowenthal, H., 47, 51, 64, 85 

Léwenthal, J., 207, 216, 217 

Léwenthal, R., 144, 211, 326, 344, 
346-351, 355, 357 

Lucchesi, 219 

Lucion, 137 

Lucius, 68 

Liideking, 32, 226 

Ludwig, 142 

Luhmann, 167 

Lummis, 191 

Lunge, 69 

Lyte, 167 


McBain, 8, 9, 10 
McCollum, 191 
McConnell, 152 
McCool, 408 
McGavack, 177, 182 
MaclInnes, 57 

van der Made, 255, 258 
Madsen, 156 

Maffia, 52 

Magee, 258 

Magnier de la Source, 47 
Mahin, 116 

Mailhe, 250 

Maisch, 306 

Majorana, 54 

Malarski, 45 
Malcolmson, 197, 198 
Malfitano, 47, 58, 54, 71, 74 
Mallet, 206 
Malyschew, 94 
Manasse, 263 

Manchot, 237 
Marawski, 295 

Mare, 259 

Marchand, 164, 301 


427 


Marchetti, 284 

Marck, 296 

Marignac, 209, 272 

Mark, 4 

Marker, 361, 363 

Marsh, 397 

Martin, F., 308, 312, 313 

Martin, G., 105, 108 

Marzano, 235 

Mascetti, 91 

Masoni, 301 

Massol, 170 

Masson, 144 

Mathews, 57 

Matschak, 254 

Matsuno, 65 

Matthews, 347, 348 

Mattson, 401, 402, 410, 416, 417 

Matula, 47, 53 

Mawrow, 151, 152 

May, 174 

Mayer, 152 

Mazzetti, 165 

Meade, 384 

Mecklenburg, 68, 204, 205, 207, 209, 
211, 212, 216, 217, 358 

Meigin, 153 

Meister, 353 

Melbye, 143 

Melikoff, 237, 259 

Mellor, 110, 189, 165 

Meneghini, 155 

Mengel, 253, 254 

Menke, 294 

Menner, 178 

Merwin, 36, 38, 73, 110, 195, 388 

Merz, 233 

Meunier, 321, 322, 334 

Meyer, J., 303 

Meyer, R. E., 131 

Meyer, R. J., 182, 250, 253, 258, 259 

Michaelis, L., 22, 65 

Michaelis, W., 166, 389, 391 

Michel, 47 

Middleton, E. B., 56, 69, 120, 122, 
126, 328 

Middleton, H. E., 397, 409 


428 


Miller, D., 146 


Miller, E. B., 127, 128, 189-191, 339, 


348, 349, 371, 375-380 
Miller, W., 145 
Milligan, 107, 109 
Millon, 173 
Mills, 114 
Minachi, 113 
Minajeff, 347 
Minot, 201 
Miolati, 91 
Mitchell, 310 
Mitscherlich, 104, 145 
Mittra, 151 
Mixer, 79 
Miyamoto, 75 
Moberg, 83, 209 
Moeller, 9, 321 
Moissan, 79, 149, 152, 219, 263 
Moles, 277 
Mondolfo, 69, 208 
Montemartini, 301 
Moore, B., 293 
Moore, T., 154 
Moraht, 161, 163, 309 
Morgan, 410, 413 
Morley, 211, 234 
Morozenwicz, 112 
Morris, 157 
Moser, 145, 146, 270, 277, 278, 289 
Mott, 71 
Mottsmith, 267 
Mouson, 414 
Mouton, 54, 55 
Muck, 35 
Mugge, 178 
Muir, 278 
Mukhopadhyaya, 57 
Mulder, 226 





Miiller, A., 115, 150, 243, 244, 246, 


254, 262, 291 
Miiller, B., 163, 358 


Miiller, E., 90, 135, 136, 143, 144, 


264 
Miiller, J. H., 200, 201 
Miiller, M., 219 
Miiller, W., 154 


THE HYDROUS OXIDES 
\ 


Mulligan, 192 
Mum, 375 
Munro, 1838 
Miintz, 407 
Musculus, 209 
Muthmann, 284 
Mutte, 357 
Mylius, 194, 307 


Nagel, C. F., 89, 90, 100, 101, 222 
Nagel, W., 284 
Nageli, 8, 55 


N 
N 


N 
N 
N 
N 
N 
N 
N 
N 





N 


apier, 336, 346 
athanson, 63 


Jeidle, 42, 43, 47, 48, 82, 83, 88, 113, 


338 
eisser, 57, 60 
ernst, 240 
euberg, 23, 169 


Neuhaus, 353 


euhausen, 23, 183 
ewall, 297 
ewhall, 393 
eyland, 196 


Nicholas, 58, 180 
Niclassen, 103, 109, 114, 159, 163, 


164, 169, 238, 240, 247, 253, 
254, 260-262 


Nicolardot, 47 
Nilsson, 129, 260 
Norcom, 378 





Nordenskiold, 227 
Nordenson, 172 

N 
Norton, 178 
N 
N 


orthcote, 89, 222 


oyes, A, A., 117 
oyes, H, A., 410 


Ober, 122 

Odén, 94 

Oechsner de Coninck, 293 
Ogata, 226 

Okazaka, 113 

Olie, 81, 83 

Olin, 414 

van Oordt, 159, 161 
Opdyke, 177, 183 


AUTHOR INDEX 


Ordway, 83, 116, 206, 209, 210 

Oryng, 67 

Osterman, 266 

Ostwald, Wilhelm, 162, 173, 174 

Ostwald, Wolfgang, 4, 17, 150, 168, 
317 

Owen, 1838 


Paal, 87, 141, 146, 154, 156, 172, 
174, 278, 288, 284, 310, 311 

Paddon, 340, 342 

Pappada, 198, 281, 290, 291 

Parenzo, 254 

Parker, 408 

Parks, 151 

Parravano, 165, 203, 209, 230, 231, 
313 

Parsell, 141 

Parsons, 159, 162, 163 

Partington, 224, 225 

Pascal, 109, 118, 169, 179, 208, 214 

Patrick, 23, 177, 181-189, 191, 192 

Patroni, 188, 299 

Patten, 164, 302 

Paucke, 68, 357 

Pauh, 7, 17, 47, 53, 115, 116, 242, 
243, 254, 256, 258 

Pavlov, 300 

Payen, 226 

Paykull, 244 

Pazuski, 163 

Péan de St. Gilles, 34, 38, 39, 104 

Pebal, 152 

Péchard, 282 

Pelet-Jolivet, 354, 363, 364 

Péligot, 83, 135, 1387 

Pelletier, 157 

Pellini, 155 

Perquin, 190 

Pfeiffer, 301 

Pfordten, 235 

Phillips, A. J., 888, 390 

Phillips, R., 34, 74, 103 

Piceini, 271, 301 

Picton, 47, 54, 94, 169 

Piefke, 368 

Piper, 10 


429 


Pirsch, 278 

Pissarjewski, 237, 246, 251 

Platzmann, 393, 395 

Pleissner, 226, 230 

Pluddemann, 69 

Podszus, 249 

Polack, 117 

Poma, 138, 299, 328 

Popp, 167 

Porter, E. C., 320 

Porter, E. E., 91, 124 

Portillo, 277 

Poser, 331 

Posnjak, 32, 36, 38, 73, 214 

Pott, 193 

Powarnin, 322 

Powis, 45 

Prandtl, 262, 265 

Prasad, 179, 180 

Prat, 157 

Preiss, 261 

Prescott, 117 

Preston, 183 

Prideaux, 277 

Procter, 3, 4, 17, 19-21, 315, 317, 
319, 328, 325, 333-335 

Proust, 48, 145 

Prud’homme, 90, 144, 161 

Puiggari, 49 

Pulfrich, 390, 391 

Purdy, 195 

Puri, 408 


Quartaroli, 139 
Quincke, 6, 55 


Rabe, 1383 

Ragozinskii, 391 
Rainer, 261 

Rakuzin, 126, 128 
Rammelsberg, 253, 273 
Ramsay, 34, 104 
Rankin, 110, 385, 387, 388 
Ransohoff, 305-807 
Rappe, 254 

Rathsburg, 309, 310 
Rauter, 141 


430 THE HYDROUS OXIDES 


Rawson, 144, 211, 344, 346-351, 355, Rodier, 292 


357 Roesti, 228 
Ray, 182, 188, 195 Rogan, 47 
Recoura, 77, 81, 83 Rogers, 118 
Reichard, 174 Rohland, 393 
Reid, E, E., 192 Rona, 56, 65 
Reid, R. D., 228 Roscoe, 208 
Reinders, 268, 291 Rose, H., 44, 156, 203, 206, 207, 209, 
Reinhardt, 69 214, 220, 233, 235, 273 
Reinitzer, 87, 88 Rose, R. P., 115 
Reinke, 32 Rosenheim, 217, 243, 280-282 
Reisig, 208 Rosenthal, 66 
Reitstétter, 128 Ross, 198 
Remy, 305, 306 Roth, 225 
Renz, 119, 131, 132, 162 Roth, D. M., 178 
Rewald, 169 Roth, K., 172 
Reychler, 67 Roth, O., 353 
Reyerson, 191 Rothaug, 79 
Reynolds, 174 Rothe, 332 
Reyonso, 208 Rousseau, 295 
Rheinboldt, 244 Rubenbauer, 117, 161, 170, 171 
Rhodes, 271 Rubens, 249 
Ricci, 301 Ruer, 47, 53, 227, 238-240, 242, 248 
Richards, 86, 131 Ruff, 35, 278, 309, 310 
Richardson, A. 8., 199 Riiger, 151 
Richardson, C., 384, 386 Ruoss, 146 
Richarz, 237 Rupp, 277 
Richter, 219, 237 Russ, 107 
Rickmann, 254 Russell, 75 
Rideal, 69 
Riedel, J. D., 265 . Sabanejeff, 193, 281, 289 
Riedel, W., 155, 156 Sabatier, 140, 154 
Rieke, 79 St. John, 188 
Riesenfeld, 155 Salkowsky, 142 
Riffard, 44 Salmon, 9 
Rindfusz, 27 Salter, 410, 413 
Rinne, 109, 111 Salvadori, 229 
Rittenhausen, 142 Salvétat, 77. 
Roard, 340 Sampson, 201 
Roberts, 159 Samsonow, 293 
Robertson, P. W., 279 Sandmeyer, 145 
Robertson, T. B., 3 Sanin, 354, 356, 365 
Robin, 45 Santesson, 272 
Robinson, F., 69 Sarason, 303 
Robinson, W. O., 47, 162, 397, 398, Saville, 370 
401, 402 Saxton, 37, 408 


Rodewald, 32 Seala, 172 


AUTHOR INDEX 


Scales, 397 

Schaffgotsch, 161 

Schaffner, 34, 76, 137, 
224, 226, 276 

Schalek, 66, 67, 102, 121, 224, 244, 
260 

Schaposchnikoff, 347, 365 

Schéele, 259 — 

. Scheetz, 71, 73, 222 

Scheibler, 292 

Schenck, 141 

Scherrer, 11, 228 

Scheurer, 245 

Scheurer-Kestner, 41, 77, 206 

Schick, 173 

Schiff, 87, 203, 217 

Schilow, 124, 300 

Schlésing, 108, 397, 406 

Schlumberger, 104, 1138, 115, 116, 
121, 339 

Schmauss, 54 

Schmidt, G., 74 

Schmidt, G. C., 357 

Schmidt, T., 37 

Schmidt-Walter, 330 

Schmitz, 201 

Schneider, E. A., 114, 214, 215, 219, 
225 

Schneider, O., 310 

Schoch, 173, 174 

Schoenfeld, 399, 416 

Scholz, 94, 95 

Sch6énbein, 237 

Schorlemmer, 208 

Schottlander, 157, 259 

Schreiner, 400, 406, 414 

Schréder, G., 151 

von Schréder, J., 316, 317, 321, 357 

von Schroder, P., 31, 32 

von Schroeder, E., 293 

Schucht, 332 

de Schulten, 75, 148, 164, 173, 302, 
358 

Schultz, 323 

Schulz, 273 

Schulze, 124, 127 

Schiirer, 164 


173, 204, 


Schuster, 268, 269 
Schwalbe, 346 

Schwarz, H., 141, 166 
Schwarz, R., 178, 179, 194 
Schwarzenberg, 152 
Schwestoff, 267 
Schwitzer, 339, 343, 349 
Scott, 204 
Sedenlinovich, 135, 136 
Seguin, 315 

Seidel, 231 

Seifritz, 267 


Sen, 98, 142, 150, 168, 278, 299 


Senderens, 154, 250, 274, 276 
Senechal, 78 
Sensburg, 156 
Serono, 276 

Serra, 178 
Seyewetz, 321, 334 
Seymour-Jones, 86, 326 
Shantz, 407 
Sharp, 396. 
Shepherd, 385 
Sheppard, 8, 180 
Shidei, 104 
Shorey, 400, 414 
Shriner, 308 
Siewert, 76, 79 
Simon, A., 275 
Simon, A. L., 180 
Simpson, 367 
Sims, 16 

Singer, 191 

Sisley, 353 
Sjollema, 301 
Skinner, 174, 301 
Slade, 117 
Smeaton, 382 
Smith, E. F., 284 
Smith, J. G., 410 
Smith, O:.M.~377 
Smith, R. B., 47 
Smoluchowski, 121 
Solstein, 295 
Sommerhoff, 334 
Sorel, 166 
Sosman, 195, 388 


431 


432 


van der Spek, 57, 60, 61 

Spencer, H. M., 377 

Spencer, J. F., 132, 258, 258 

Spiro, 17 

Spitzer, 135, 136 

Spring, 134, 137, 295, 299 

Stallo, 276, 277 

Stapelfeldt, 267 

Steele, 144 

Steiner, 331 

Steinmetz, 249 

Sterba, 253, 258 

Stericker, 196-199 

Stevens, 246, 247 

Stevenson, 178, 273 

Steyer, 311 

Stiasny, 22, 316, 321 

Stingl, 295 

Storch, 220, 221 

Stéwener, 179, 194 

Stiibel, 9 

Suida, 116, 339, 343, 345, 358, 359 

Sullivan, 301 

Sulman, 199 

Sundell, 69 

Swan, 249 

Sweet, 180 

Swiontkowski, 296 

Szegvary, 66, 67, 102, 121, 224, 244, 
260, 268, 

Szilard, 142, 230, 244, 247, 262, 292 


Tacchini, 154 
Tagliani, 350 
Tanatar, 155 
Tatters, 167 
Taylor, Hy Gad 
Taylor, H. 8., 69 
Taylor, W. A., 189 
Teichmann, 152 
Thenard, 340 
Thibault, 277 
Thiel, 131, 1382 
Thiele, 178 
Thieler, 108 
Thierault, 371, 373, 374, 377, 380 
Thomann, 353 


THE HYDROUS OXIDES 


Thomas, 47, 48, 63, 317, 320, 322, 
324, 325, 328-330, 334 

Thompson, 325 

Thomsen, 312 

Thomson, F. C., 7 

Thomson, J., 297 

Thorpe, 45, 69, 308 

Thuau, 334 

Thuring, 395 

Tieri, 55 

Tingle, 346 

Tippermann, 392 

Tommasi, 34, 103, 104, 116, 137, 188 

TopsGe, 312 

Tower, 150, 152, 154 

Traube, 144, 237 

Traube-Mengarini, 172 

Traver, 249 

Travers, 297 

Tremper, 393 

Tressler, 200 

Tribot, 43 

Trillat, 298, 303 

Truog, 396, 418, 414 

Tschermak, 178, 206, 217 

Tschugaev, 154 

Tubandt, 150, 155, 156 

Tischer, 135, 140, 316 

Tuttschew, 233 


Ufer, 47, 48 
Ullick, 281 
Ulrich, 393 
Urbain, 191, 252 
Urban, 178, 184 
Utescher, 358 
Utz, 345 

Utzino, 120 


Vail, 198 
Vanino, 157, 278 
Van’t Hoff, 237 
Vanzette, 178 
Varet, 173 
Vegesack, 289 
Veil, 155 
Venable, 238 


AUTHOR INDEX 


Verneuil, 47, 111, 247, 250, 253, 258 
Vervloet, 291 

Vespignani, 118 

Vesterberg, 260 

Vignon, 210 

Villiers, 185, 137, 303 

Vincent, 76 

Vogel, 158 

Vorlander, 128 

Voss, 156 


van der Waals, 5 

Wachter, 155 

Waegner, 254, 260, 261 

Wagner, C. L., 66, 72, 332 

Wagner, R. F., 234 

Walden, 193 

Walker, C. H. H., 204 

Walker, J., 104, 131, 133, 155, 173, 
204, 238, 253, 186 

Walpole, 11, 4138 

Walter, 47 

Washburn, 32 

Waterman, H. I., 190 

Waterman, H. J., 301 

Waw, 191 

Weaver, 103, 109 

Weber, B., 118, 120 

Weber, R., 204, 209 

Webster, 293 

Wedekind, 244, 245 

Wedenhorst, 118 

Weeren, 162 

Wegelin, 264, 265, 290 

von Weimarn, 9, 138, 15, 23-27, 94, 
118 

Weinland, 87, 88 

Weiser, 8, 28-30, 39, 41, 56, 58, 62, 
65, 69, 70, 80, 81, 87, 91, 94, 97, 
99, 109, 112, 113, 115, 120, 122, 
125-127, 138, 144, 210, 212, 
213,217, 221, 222, 224, 283, 
300, 328, 415 

Weiss-Landicker, 272 

Weller, 237 

Welsbach, 240, 249, 250, 259 

Weltzien, 108 


433 


Wengraf, 245 

Werner, 86, 87, 142, 193, 327, 349 

Wernicke, 230 

Werther, 133 

Weston, 370 

White, 167, 249 

Whiteley, 347 

Whitman, 75 

Whitner, 153 

Whitney, 117, 122, 396, 399 

Whittaker, 348, 350 

Wick, 155 

Wiedemann, 32 

Wiegner, 399 

Wiessmann, 306 

Wigner, 268 

Wiley, 403 

Wilhelms, 237 

Wilke, 245 

Williams, 191, 332, 397 

Williamson, 339, 349, 361, 379 

Willstatter, 103, 104, 105, 205 

Wilm, 307 

Wilson, J. A., 17, 19-21, 317, 320, 
323, 325, 328, 329 

Wilson, R. E., 188 

Wilson, W. H., 19 

Windhausen, 77, 78 

Wingraf, 338 

Winkelblech, 147, 226, 227, 247 

Winkler, 199, 200 

Wintgen, 49, 51, 64, 85, 86, 210, 
326 

Wischin, 309 

Wislicenus, 105, 327, 357 

Witt, 253, 254 

Witteveen, 149, 151 

Wittstein, 157, 206, 244 

Witzemann, 298, 301 

Witzmann, 311 

Wobling, 209 

Wohler, F., 75, 108, 280, 272 

Wohler, L., 69, 71, 77, 79, 81, 154, 
219, 220, 239, 281, 289-291, 
308, 309, 311-313 

Wohler, P., 69 

Wolff, 32 


434 


Wood, 90, 118, 161, 171, 207, 210, 
211, 214, 217, 218, 226, 234, 277 

Woodward, 278 

Worsely, 279 

Wosnessensky, 42 

Woudstra, 88, 89 

Wright, C. R. A., 294 

Wright, F. E., 385 

Wright, L. T., 42 

Wyrouboff, 47, 247, 250, 253, 258 


Yamada, 208, 214 


THE HYDROUS OXIDES 


Yanek, 120 
Yoe, 71, 222 


Zambonini, 260 

Zedner, 155 

von Zehman, 105 

Zimmerman, 152 

Zocher, 216, 266-269 

Zsigmondy, 5-8, 11, 14, 40, 46, 53, 
85, 117, 178, 194; 195,32: 
214, 215, 219, 220, 228, 273 

Zunino, 104 


SUBJECT INDEX 


A 


‘‘Acclimatization,”’ 69, 70 
Adsorption (see also this heading 
under several hydrous oxides). 
capillary theory of, 183-185 
effect of neutral salts, 328 
influence of hydrogen ion concen- 
tration, 91-94, 316, 320, 321, 
324 
isotherms, 185, 186, 286, 324, 330, 
331, 341, 355, 360 
maxima in, 331, 332 
mutual (see Mutual adsorption). 
preliminary to chemical reaction, 
245 
reversibility, 332 
theory of composition of sol, 48, 
52, 53 
of mechanism of mineral tan- 
ning, 325, 328 
selective, in the soil, 409-411 
Agar jellies, 5, 8, 12 
Agate, 181 
Agglomeration, prevention of, 71, 73 
Albumin, 16, 65, 68, 70, 128, 298, 
303, 334 
adsorption by hydrous alumina, 
128 
of arsenious acid, 68 
sol, 65, 70 
mutual precipitation of ferric 
oxide sol and, 65 
swelling, 16 
Albumin-ferric oxide sol, 65 
Alizarin, 268, 336, 358-361 
adsorption of, 358-361 
iron-alizarin lake, 359, 360 
streaming double refraction, 268 


Alum as coagulant in water purifi- 
cation, 370-380 
Alumina mordant, 338-347 
Aluminum oxide, anhydrous, 110- 
112 
corundum gems, color of, 111, 112 
modifications, 110 
Aluminum oxide, hydrous, 103-129, 
162, 285, 319, 337-346, 356, 
361-363, 365, 369-380 
adsorption by cotton, 345 
wool, 340 
adsorption of acid and basic dyes, 
361-363 
effect of hydrogen ion concen- 
tration, 362, 363 
adsorption of arsenious acid, 68 | 
albumin, 128 
alizarin, 358, 361 
ammonium ion, 406 
calcium, 403, 404 
casein, 128 
chondrin, 128 
chromate, 123, 126 
Congo blue, 128 
red, 364 
ferrocyanide, 128 
gum arabic, 126 
hide, 333 
magnesium, 405 
phosphate, 405 
potassium, 405 
precipitating ions, 122-124 
order of, 123, 124 
sulfate, 405 
tannin, 357 
tuberculin, 128 
ageing, 109, 160 
composition, 103-107 


435 


436 


Aluminum oxide, hydrous, fibrous, 
105 
floc, 371-380 
glow phenomenon, 110 
in dyeing, 338-347 
in intestinal infections, 129 
in purification of diphtheria anti- 
toxin, 129 
of pepsin, 129 
of water, 370-380 
jellies, 121 
mordant for cotton, silk, and wool, 
339-346 
reversibility of precipitation, 125- 
126 
sol, 112-122 
action of ammonia and alkalies, 
116-119 
formation of aluminate, 117, 
118 
coagulation, 120, 121 
mechanism of, 121 
velocity, 121 
composition, 115, 116 
formation, 112-120 
in presence of glucose, 120 
precipitation values of electro- 
lytes, 122, 123 
sol-gel transformation, 121 


temperature-dehydration curve, 
105-107 
Aluminum oxide, trihydrate, 107- 
110, 118 


preparation, 108 
x-ray analysis, 109, 110 
Amethyst, artificial, 111 
manganese oxide in, 302 
Anaemia, treatment of, 45, 67, 200 
germanium dioxide in, 200 
intravenous injection of ferric 
oxide 45, 67 
Aniline blue, streaming 
refraction of, 268 
Antagonism of ions, 94, 98 
Antimony pentoxide, hydrous, 274- 
276, 337 
adsorption of alkali salts, 275 


double 


THE HYDROUS OXIDES 


Antimony pentoxide, hydrous, ad- 
sorption of phosphoric acid, 
276 
gels, 276 
optical phenomena during dehy- 
dration, 275 
sol, 276 
vapor-pressure isotherms, 275 
sulfide sol, 63 
tetroxide, 277 
_trioxide, hydrous, 276, 277 
Arsenate jellies, 11, 27-29 
Arsenic poisoning, hydrous ferric 
oxide as antidote, 45, 67, 68 
Arsenious acid, 45, 67, 68, 163, 245, 
337 
adsorption of, 45, 67, 68, 245 
solid solution with beryllium 
oxide, 163 
Arsenious sulfide sol, 63, 331 
‘“‘acclimatization,”’ 70 
adsorption of alkalies, 99, 124, 125 
of precipitating ions, 62, 125 
order, 125 
precipitation by electrolytes, 57, 
61, 62 
effect of concentration of sol, 
57, 62 
stabilizing ions, 61, 62 
precipitation by mixtures of elec- 
 trolytes, 97-99 
factors determining, 97, 99 
Artificial gems, 110-112, 141, 302 
vegetation, 197 
Assistants, mordanting, 343, 348, 
349 
Auric oxide, hydrous, 156, 157 
Aurous oxide, hydrous, 157, 158 
sols, 157, 158 


B 


Bacteria in soils, 401 
Barium, malonate jellies, 8, 11 
sulfate, adsorption of selenium 
oxychloride by, 15 
gelatinous, 15, 25 
jellies, 25,29 


SUBJECT INDEX 


Battery, Edison (see Edison storage 
batiery). 
Le Clanche, manganese dioxide in, 
302 
Bauxite, 109 
Benzene, silica gel in recovery of, 
190, 191 
Benzopurpurine, gelatinous crystals, 
13 
jellies, 11, 13 
streaming double refraction, 268 


Beryllium hydroxide, crystalline, 
160-162 
effect of heat on, 160 
pure, 162 


solubility in salt solutions, 162 
oxide, anhydrous, 163, 164 
uses, 163, 164 
oxide, hydrous, 159-164, 170, 334 
adsorption by, 163 
ageing, 159, 161 
solid solution with arsenious 
oxide, 163 
with boric acid, 163 
transformation to crystalline, 
- 159 
_ x-ray analysis ,159 
Bismuth hexoxide, 279 
tetroxide, 278, 279 


trioxide, hydrous, pure, 277, 278 © 


in ceramics, 278 
sols, 278 
Bleriot lamp, 240 
Boric acid, adsorption of, 163 
solid solution with beryllium ox- 
ide, 163 
Bricks, ferric oxide in, 71, 73 
Brine, effect on silicate of soda, 197 
Bromine in tanning, 334 


C 


Cadmium hydroxide, 172 

jellies, 15 

oxide, hydrous, 172, 173 
Calcification in tuberculosis, 196 
Calcium, adsorption by silica gel, 177 

by soil colloids, 403, 404 


437 


Calorescence (see Glow phenomenon). 
Carbon, adsorption by, 188, 316, 331 
Casein, adsorption of, 128 
Catalyst, ferric oxide as, 69, 70 
nickel oxide as hydrogenation, 
153, 154 
silica gel, 192 
thorium dioxide, 250 
uranium dioxide, 293 
vanadium pentoxide, 270 
Cellular or honeycomb theory of 
jelly structure, 5-8 
Cellulose jellies, 12 
Cement, 382-395 
aluminous, 394, 395 
dental, 164, 172 
glass, silicate of soda as, 199 
iron-Portland, 393, 394 
magnesia (see Magnesia cement). 
Portland, 382-393 
CaO-Al1,03-SiO2 
387 
clinker, 387 
composition, 384-389 
dicalcium silicate, 390-391 
tricalcium aluminate, 390 — 
tricalcium silicate, 391 
discovery, 383 
function of gypsum in, 384, 392 
manufacture, 383, 384 
setting and hardening, 389, 390 
effect of calcium chloride, 393 
effect of salts, 392, 393 
theory, 393 
Pozzolana, 383 
Centrifugal methods of preparing 
sols, 43 
Ceramic pigments (see Pigments). 
Ceric oxide, hydrous, 252-258, 280, 
285, 337 
color, 253 
jellies, 255, 298, 299 
mordant, 254 
sol, 62 
action of electrolytes, 255 
of radium rays, 256, 257 
ageing, 255 


system, 386, 


438 


Ceric oxide, hydrous, sol, viscosity- 
time curve for, 255, 256 
in tanning, 254 
in Welsbach mantel, 254 
x-ray analysis, 253 
Cerium peroxide, 254 
Cerous oxide, hydrous, 258, 259 
Chance-Claus process, 69 
Charcoal, adsorption of arsenious 
acid, 68 
Chlorine in tanning, 334 
Cholic acid, gelatinous crystals, 13 
Chondrin, adsorption by hydrous 
alumina, 128 
Chromate, adsorption by hydrous 
alumina, 123, 126, 327 
Chrome green, 80 
mordant, 335, 347-350 
Chromic acetates, complex, 87, 88 
Chromic oxide, hydrous, 30, 31, 76- 
102, 222, 285, 319, 337, 338, 358 
adsorption by hide, 324-327 
by, influence of hydrogen ion 
concentration on, 91, 94 
of alizarin, 358 
other hydrous oxide, 90, 144 
oxalate, 91-94, 96 
ageing, 77, 78, 160 
alkaline solution, colloidal nature, 
78, 89, 90 
color, 80-82 
factors determining, 81, 82 
composition, 76, 77 
glow phenomenon, 78-80 
cause of, 79, 80 
jellies, 27, 91, 100-102 
from negative sol, 101 
positive sol, 91 
mordant for cotton, silk, and wool, 
347-351 
peptization by alkalies, 89, 90 
pigment, 76, 77, 80, 81 
chrome green, 80 
Guignet’s green, 76, 80, 81 
sol, 57, 59, 82-100, 116 
composition, 83-86, 88 
intermittent dialysis, 83 


THE HYDROUS OXIDES 


Chromic oxide, hydrous, sol, mole- 
cules in micelle, number from 
membrane potential measure- 
ments, 84-86 

negative, 89, 100, 116 
precipitation by mixtures of 
electrolytes, 94-95 
factors determining, 97, 99 
_ values of salts, 91, 100 
preparation, 82-89 

Chromic oxide, in gems, 111, 112 

Chromic sulfate, basic, 86 

Chromite, 90 

Chrysophenene, gelatinous crystals, 


13 
jellies, 11, 13 
Clay, colloidal, precipitation by 
electrolytes, 415, 416 
slip, 199 | 
streaming double refraction of, 
268 


Coagulents in water purification, 
375-381 
Cobalt blue, 150 
glass, cause of color, 151 
green, 150 
Cobaltic oxide, hydrous, 151, 152 
Cobalto-cobaltic oxide, hydrous, 152 
Cobaltous hydroxide, 148-150 
Liesegang rings of, 150 
plechroism in, 149 
x-ray analysis, 148 
oxide, anhydrous, 150, 151 
ceramic pigment, 150 
dryer for paints, 151 
oxide, hydrous, 90, 147-151, 222 
color, 147-151 
change from blue to rose, 
effect of nickel salt on, 148 
sols, 150 
solubility in alkali, 150 
Collagen of hide, 319-321 
adsorption by, 319, 320 
isoelectric point of, 320, 321 
Colloidal and molecular solutions, 
287 
distinction between, 287 


SUBJECT INDEX 


Colloidal forest, 197 
matter in soils, 396-418 
in surface waters, 366 
Columbium pentoxide, 272, 273 
separation from tantalum pent- 
oxide, 273 
sol, 272 
Composition of sols, complex theory 
of, 47, 83, 207 
Congo blue, adsorption of, 128 
red, 336, 364 
adsorption of, 163, 364 
Contact process for sulfuric acid, 191 
Corrosion of iron, 75 
Corundum gems, pigments in, 110- 
112, 141, 302 
rubies, 111, 112 
sapphires, 111 
amethyst, 111, 302 
emerald, 111, 141 
oriental topaz, 111 
Cotton, mordants for, 345, 346, 
350-352, 354 
adsorption of molybdenum blue 
by, 286 
Crystal violet, adsorption of, 331 
Cupric oxide, anhydrous, in artificial 
emeralds, 141 
ceramic pigment, 141 
Cupric oxide, crystalline, 135, 136, 
160 
Cupric oxide, hydrous, 90, 134-145, 
222, 277, 337, 364 
adsorption by hydrous chromic 
oxide, 144 
of eosin, 364 
effect of salts on, 364 
color, 140, 141 
composition, 134, 139 
' dehydration, 136, 189, 141 
darkening during, effect of 
alumina, 141 
mechanism of, 136, 139 
jellies, 144, 145 
effect of sulfate on, 145 
sols, 141-144 
coagulation, 142, 143 


439 


Cupric oxide, hydrous, sols, coagula- 
tion, effect of stirring, 142, 


143 
‘“‘discharge electrolysis,” 142 
fungicidal action, 143 
preparation, 141-143 
stabilizing agents for, 141- 


143 
solubility in alkalies, 148, 144 
effect of tartrate, glycerin, and 
mannite on, 143, 144 
spontaneous dehydration of, 311 
stability of blue, 137-140 
effect of alkalies, 137 
of hydrogen peroxide, 139 
of salts, 137-139 
Cuprous oxide, anhydrous, as ce- 
ramic pigment, 146 
in antifouling paints, 146 


Cuprous oxide, hydrous, 145-147 
sols, 146, 147 
D 
Deflocculating action of silicate 


of soda, 199 
Deflocculation of soils, 414-418 
Dehydration curves, 38, 106, 107, 
175, 176, 288, 289 
Dental cement, beryllium oxide in, 
164 
zine oxide in, 172 
Dialysis of sols, 27, 28, 43, 83 
electro, 43 
intermittent, 83 
Di-benzoyl-l-cystine jellies, 9 
Dimolybdenum pentoxide, hydrous, 
282, 283 
color, 282 
jelly, 283 
oxidation to molybdenum blue, 
283 
Diphtheria antitoxin, hydrous alu- 
mina in purification of, 129 
“Discharge electrolysis,” 142 
Disinfection in water purification, 
367, 370 


440 


Donnan theory of membrane equili- 
bria, 17-22, 318, 319 
application to swelling of gela- 
tin, 19-22 
Drummond light, 240 
Dyeing, mordants in (see Mor- 
dants). 
Dysprosium oxide, hydrous, 262 
E 
Edison storage battery, hydrous 
nickel peroxide in, 155 
nickelous hydroxide for, 153 
Elasticity of silica gel, 180 
of vanadium pentoxide sols, 267 
Electro dialysis, 43 
Electrometric measurements, ad- 
sorbed chloride in, 243 
Emerald, artificial, 111, 141 
Emulsion theory of jelly structure, 5 
Emulsions with silica gel, 195 
Eosin, adsorption of, 364 
“‘Equivalent aggregate,” 48, 51, 84- 
86 
Erbium oxide, hydrous, 262 
jelly, 262 
sol, 262 
Europium oxide, hydrous, 261, 262 


F 


Fehling’s solutions, nature of, 144 
Fermentation, alcoholic, manganese 
dioxide in stimulating, 301 
Ferric oxide-albumin sol, 65 
Ferric oxide, hydrous, 13, 30, 31, 34- 
74, 90, 192, 222, 263, 282, 285, 
319, 337, 338, 358, 361-363, 
369, 381 
“acclimatization ”’ of, 69, 70 
adsorption of acid and basic dyes 
by, 361, 362 
alizarin, 359, 360 
ammonia, 400 
and hydrous stannic oxide, 
mutual, 221 


THE HYDROUS OXIDES 


Ferric oxide, hydrous, adsorption of 
arsenious acid, 45, 67, 68 
effect of hydrogen ion concen- 
tration on, 361, 362 
magnesium, 405 
phosphate, 405 
potassium, 405 
precipitating ions, 69, 70 
sodium hydroxide, 359 
sulfate, 405 
agglomeration prevented by 
alumina, 71, 73 
antidote for arsenic poisoning, 45, 
67, 68 
brown, 71-73 
catalytic action, 68, 69 
color, 34, 35, 44, 70-74 
effect of size of particles, 71-73 
pigment in bricks, 71, 73 
composition, 34-38 
fractional precipitation, 70 
jellies, 65-67 
by dialysis, 67 
by precipitation of sol, 66 
sol-gel transformation, 9, 11, 16, 
66, 121 
mordant for cotton, silk, and wool, 
351, 352 
red, 34, 71-74 
relation to yellow, 34, 35 
sols, 38-65, 193, 221, 244, 283 
color, 44, 45 
composition, 46—54, 86 
adsorption theory of, 48, 52, 53 
complex theory of, 47 
purity, 48 ji 
effect of dextrose on freezing 
point, 46 
~ “equivalent aggregate,’ 48, 52 
for intravenous injection in 
anaemia, 45 
freezing-point lowering, 53, 54 
Graham’s, 42-44, 291 
mutual precipitation of and 
other sols, 62-65 
negative, 44—46 
protective colloids for, 44, 45 


SUBJECT INDEX 


Ferric oxide, hydrous, sols, optical 
properties, 54, 55 
Majorana phenomenon, 54, 55 
osmotic pressure, 53, 54 
Péan de St. Gilles’, 38-42, 58, 
291 
types of precipitates from, 39, 
40 
yellow, 41 
precipitation of by electrolytes, 
55-62 
effect of concentration of sol, 
57-62 
effect of stabilizing ion, 61, 62 
heat effect, 57 
order of ions, 56 
precipitation values of electro- 
lytes, 50-58 
factors determining effect of 
concentration, 60-62 
preparation, 39, 41-45 
relation of yellow to red, 46 
sensitivity of, effect of non-elec- 
trolytes on, 65 
streaming double refraction, 54, 


268 

sugar, effect on crystallization of, 
tf 

temperature-composition curves 
of, 38 


x-ray analysis, 37, 38 
yellow, 34, 35, 41, 71-74 
stability, 73, 74 
Ferric oxide, in gems, 112 
Ferric oxide, monohydrate, 36, 73 
Ferro-ferric oxide, hydrous, 75 
Ferrous oxide, hydrous, 74, 75, 380, 
381 
and rate of corrosion of iron, 75 
in estimating nitrites and nitrates, 
75 
_ sulfate as coagulent in water puri- 
fication, 380, 381 
Fibers, adsorption of dyes by, 331 
Fibrillar structure of jellies, 8-10, 12 
Fibrin jellies, 11 
swelling of, 16 


441 


Filtering agent, silica gel as, 192 
Filtration in water purification, 367- 
381 
mechanical, 369-381 
slow sand, 367-369 
Fire brick, use of magnesia in, 166 
Fixing agents for mordants, 356-358 
theory of action, 356-358 
Floc, alumina, 317-380 
ferric oxide, 380-381 
Flocculation of soils, 414-418 
Forest, colloidal, 197 
Formaldehyde, catalyst in synthesis 
of, 293 : 
in tanning, 322 
Fuller’s earth, adsorption of base by, 
409, 411 


G 


Gadolinium oxide, hydrous, 261, 262 
Gallium oxide, hydrous, 129, 130 
action of alkalies, 129, 130 
ageing, 130 
composition, 130 
formation, 129 
gelatinous character, 130 
Gases, adsorption by silica gel, 181- 
184 
Gelatin, 3-33, 147, 290, 303, 316 
jellies, 3-33 
sol, 13, 14, 64 
mutual precipitation of ferric 
oxide and, 64 
swelling, 16-23 
application of Donnan theory of 
membrane equilibrium, 19-22 
effect of hydrogen ion concen- 
tration, 17 
of neutral salts, 21 
Procter- Wilson theory, 19-22 
reversibility, 22 
x-ray analysis, 11 
Gelatinous crystals, 18, 14 
of benzopurpurine, 13 
of cholic acid, 13 
of chrysophenene, 13 


442 


Gelatinous precipitates, 3-33 
conditions favoring formation, 
26 
structure, 13 
Gelation, micellar orientation in, 10, 
12 
Gels, 3, 9, 15, 16, 26-31 
elastic, 3, 9, 16, 31 
forms, 3 
non-aqueous, 23 
non-elastic, 3 
preparation, 15-80 
structure, 3-15 
vapor pressure, 30-33 
von Weimarn’s theory of forma- 
tion, 24, 25 
Gems (see Artificial and corundum 
gems). 
Germanium chloroform, 201 
dioxide, hydrous, 199-201 
forms, 200 
in treating anaemia, 201 
Germanous oxide, hydrous, 201 
Gibbsite, 109 
Glass, cement for, silicate of soda as, 
199 
manganese dioxide as decolorizer 
for, 302 
stains for (see Pigments, ceramic). 


Glow phenomenon, 78-80, 110, 233, 


234, 239, 273, 309 
Gold sol, 63, 72, 94, 219 
color, 72 
Gothite, 35, 36 
Guignet’s green, 76, 80, 81 
Gum arabic, 128, 147, 298 
adsorption by hydrous alumina, 
128 
Gypsum in cement, 384, 392 


H 


Hardening of cement, 389-393 
Hardpan, formation of, 418 
Hargreaves-Robinson process, 69 
Hematite, 35, 36 

as pigment, 70 

color, 70, 71 


THE HYDROUS OXIDES 


Hexavanadic acid, 263 
Hide, 314-335 
adsorption of hydrous chromic 
oxide, 324-330 
effect of hydrogen ion con- 
centration, 324 
effect of neutral salts, 328 
reversibility, 332 
of sulfuric acid, 328 
of tannin, 316-320 
effect of hydrogen ion concen- 
tration, 316, 320, 321 
preparation of for tanning, 314 
Holmium oxide, hydrous, 262 
Honeycomb or cellular theory of 
jelly structure, 5-8 
Humic acid in soils, 400 
Hydrogen ion concentration, effect 
on adsorption by hydrous chro- 
mic oxide, 91-94, 324 
formation of alumina floc, 372-375 
of acid and basic dyes by hydrous 
alumina, 362, 363 
of acid and basic dyes by hydrous 
ferric oxide, 361-363 
of tannin, 316, 320, 321 
on swelling of gelatin, 17 
Hydrolysis, preparation of gels by 
slow, 29 
of indium monoiodide in air, 132 
of sols by, 87-89, 112, 113, 142, 
194, 216, 265 
Hysteresis in dehydration of silica 
gel, 177 : 
of tantalum pentoxide gel, 273 


Lei 


Indium monoiodide, hydrolysis in 
air, 132 
oxide, hydrous, 181, 1382. 
action of alkalies and ammonia, 
131 
ageing, 131, 132 
- composition, 131 
sol, 132 
Intermittent dialysis, 83 


SUBJECT INDEX 


Intestinal diagnosis, thorium dioxide 
in, 250 
zirondium dioxide in, 241 
infection, hydrous alumina in, 129 
Tonic antagonism, 94, 98 
Iridium dioxide, hydrous, 311, 312 
color, 311 
sol, 311 
sesquioxide, hydrous, 310, 311 
spontaneous dehydration, 310 
Iron corrosion, hydrous ferrous oxide 
and rate of, 75 
mordants, 351, 352 
weighting of silk with, 352 
tanning, 333 


Jellies, 3-33 
agar, 5, 8, 12 
aluminum oxide, 30, 121 
antimony pentoxide, 276 
arsenate, 11, 27, 28, 29 
barium malonate, 8, 11 
sulfate, 25, 26 
benzopurpurine, 11, 13 
cadmium, 15 
cellulose, 12 
_ ceric oxide, 255, 298, 299 
chromic oxide, 27, 91, 100, 102 
chrysophenene, 11, 13 
cupric oxide, 144, 145 
di-benzoyl-l-cystine, 9 
erbium oxide, 262 
ferric oxide, 65-67 
fibrin, 11, 16 
formation, by dialysis of sols, 27, 
28 
by metathesis, 23-30 
by precipitation of sol, 26, 27 
conditions favoring, 26 
effect of presence of salts on, 26 
of rate of precipitation on, 28, 
29 
from negative sol, 101 
from positive sol, 91 
gelatin, 3-33 


443 


Jellies, manganese dioxide, 298, 299 
mercuric oxide, 174 
molybdenum pentoxide, 283 

trioxide, 280 

nickelous hydroxide, 154 
scandium oxide, 260 
silica, 5, 6, 8, 11, 12, 175-1938, 197 
soap, 8, 10 
stability, 7, 8 
stannic oxide, 11, 224, 273 
starch, 226 
structure, 3-12 

cellular or honeycomb theory, 

5-8 

emulsion theory, 5 

fibrillar, 8-12 

micellar or sponge theory, 8-12 

solid-solution theory, 4 
swelling of, 16, 32, 33 

pressures, 32, 33 
titanium dioxide, 11 
vapor-pressure relations, 30-33 
zirconium dioxide, 243, 244 


L 


Lake, color, 336, 358-365 
iron-alizarin, 359, 360 
iron-methylene blue, 363 

Lanthanum oxide, hydrous, 260 
sol, 260 

Le Clanche battery, 302 

Lead monoxide, crystalline, 227, 228 
color, 228 
forms, 227 
polymorphism, 228 
x-ray analysis, 228 | 

Lead monoxide, hydrous, 225-230 
color, 226 
composition, 226 
mordant, 229 
mutual adsorption of and hydrous 

thorium oxide, 230 

Lead peroxide, hydrous, 230-232 

sesquioxide, 231 
sol, 231 
action of electrolytes, 231 


444 


Lepidocrocite, 35, 36 
Liesegang rings, in silica gel, 181 
of cobaltous hydroxide, 150 
of magnesium hydroxide, 

189 
theory of formation of, 168 
Limnite, 35, 70 
Limonite, 35, 36, 70 
Litharge, 232 
Lutecium oxide, hydrous, 262 


M 


167, 


Magenta, adsorption by tannin, 365 
Magnesia cement, 166, 167 
setting of, 166 
Sorel’s, 166 
application, 167 
use in manufacturing fire brick, 
166 
in mortar making, 166 
Magnesium hydroxide, 
164-169, 302 
rhythmic bands, 167-169 
sols, 169 
x-ray analysis, 164 
Magnesium oxide, anhydrous, 165- 
167 
effect of on concrete, 167 
hydration of, 165 
effect of ignition temperature 
on rate, 165 
Magnesium oxide, hydrous, as an 
antacid, 167 
as clarifier in refining of sugar, 167 
Magnetic analysis, of hydrous stan- 
nic oxide, 214 
of silica gel, 179 
Magnetite, as pigment, 71 
Majorana phenomenon, 54, 266-269 
Manganese dioxide, decolorizer for 
glass, 302 
dryer for oils, 302 
hydrous, 90, 222, 294-302, 337 
adsorption of hydroxyl ion, 
294, 295 
causes hydrolysis of neutral 
sols, 294, 295 


hydrous, 


THE HYDROUS OXIDES 


Manganese dioxide, hydrous, color, 
294 
effect on enzymic activity, 301 
in stimulating alcoholic fer- 
mentation, 301 
growth of plants, 301 
metabolism, 301 
jelly, 298, 299 
oxygen carrier, 303 
sols, 295-299 
adsorption of ions during 
precipitation, 299 
catalytic decomposition of 
hydrogen peroxide, 296, 
297 
preparation, 295, 297, 298 
vortex rings with, 297 
in amethyst, 302 
in Le Clanche battery, 302 
Manganic oxide, hydrous, 303 
Mangano-manganic oxide, hydrous, 
303, 304 
color, 304 
Manganous hydroxide, 302 
oxide, hydrous, 302, 303 
sol, 302, 303 
Mars pigment, 71, 73 
Mastic sol, 57 
Membrane equilibria, Donnan the- 
ory of, 17-22, 318, 319 
application to swelling of gelatin, 
19-22 
potential, 18, 19 
equation for, 19 
measurements, 84, 86 
Mercuric oxide, crystalline, 173, 174, 
228 
color, 173, 174, 228 
effect of size of particles, 
173 
hydrous, 173, 174 
jellies, 174 
sol, 173 
Mercurous oxide, hydrous, 174 
Metathesis, formation of jellies 
by, 23-30 
Methylene blue-iron lake, 363 


SUBJECT INDEX 


Micellar or sponge theory of jelly 
structure, 8-12 
orientation in gelation, 10, 12 
Micelles, chromic oxide sol, 84-86 
determined by membrane-poten- 
tial measurements, 84-86 
“equivalent aggregate,” 51, 84 
ferric oxide sol, 48—52 
determined by electrical methods, 
48-52 
number of molecules in, 48-52, 
84-86 
Milk of magnesia, 167 
Mineral tanning, 322-335 
Minerals, iron oxide, 35 
Minium, 231, 232 
Mixture of electrolytes, precipita- 
tion of arsenious sulfide sol by, 
97-99 
chromic oxide sol by, 94, 95, 97, 99 
Molecular and colloidal solutions, 
287 
distinction between, 287 
Molybdenum blue sol, 62, 244, 284— 
288 
adsorption isotherms, 286 
as dye bath, 285-287 
mutual precipitation of and 
other hydrous oxides, 285 
dioxide, hydrous, 283 = 
pentoxide (see Dimolybdenum pen- 
toxide). 
sesquioxide, 284 
trioxide, hydrates, 30, 31, 280 
x-ray analysis, 280 
hydrous, 280-282 
jelly, 280 
protective action on tungsten 
trioxide, 290 
sol, 280-282, 289. 
Mordanting, theory of, 349 
assistants, 343, 348, 349 
of cotton with alumina, 345, 346 
with chrome, 350, 351 
with iron, 352 
with tin, 210, 354 
of silk with alumina, 344, 345 


445 


Mordanting, of silk with alumina, 
344, 345 
with chrome, 350 
with iron, 351 
with tin, 210, 353, 354 
of wool with alumina, 339-344 
assistants in, 343 
with chrome, 345-350 
assistants in, 348, 349 
with iron, 351 
with tin, 210, 352 
fixing of, 353 
Mordants, 336-365 
acid, 337 
alumina, 338-347 
basic or metallic, 337 
ceric oxide, 254 
chrome, 347-350 
fixing agents for, 356-358 
theory of action of, 356-358 
for cotton, silk, and wool (see 
Mordanting). 
Iron, 351, 352 
tannin, 354, 356 
tin, 210, 352-354 
titanium dioxide, 235 
vanadium pentoxide, 271 
zirconium dioxide, 245 
Mutual adsorption of hydrous chro- 
mic and other oxides, 90, 144 
of hydrous stannic and ferric 
oxides, 221 
of hydrous thorium oxide and 
lead monoxide, 230 
precipitation of sols, 62-65 
ferric oxide and albumin sol, 65 
and gelatin sol, 65 
and other sols, 62—65 
mechanism of, 63, 64 
molybdenum blue and other sols, 
285 


N 
Negative sols, 44, 45, 89, 100, 101 
formation of jellies from, 101 


Neodymium oxide, hydrous, 
261 


5 


260, 


446 


Neodymium oxide, hydrous, color, 
261 
reflection spectrum, 261 
sol, 261 
Nickel oxide, anhydrous, 152, 153 
catalytic agent for hydrogen- 
ation, 153 
ceramic pigment, 153 
color, 152 
peroxide, hydrous, 155 
in Edison storage battery, 155 
suboxide, 153-154 
Nickelic oxide, hydrous, 155 
Nickelous hydroxide, 152, 153 
for Edison storage battery, 
153 
jelly, 154 
sol, 154 
oxide, hydrous, 90, 152-155, 222 
Night blue, adsorption of, 331 
Nitrates, adsorption by zine oxide, 
170 
by soil colloids, 403 
estimation with ferrous oxide, 75 


Nitrites, estimation with ferrous 
oxide, 75 
Nitrogen fixation, silica gel in, 191 


O 


Oil, dryer for, manganese dioxide as, 
302 
vanadium pentoxide as, 271 
Opacifying agent, zirconia as, 241 
Optical phenomena during dehydra- 
tion, 175, 275 
properties of sols (see Majorana 
phenomenon). 
Osmium dioxide, hydrous, 309, 310 
glow phenomenon, 309 
sol, 310 
monoxide, hydrous, 309 
tetroxide, 310 
stain for biological prepara- 
tions, 310 
Osmotic pressure of ferric oxide 
sols, 53, 54 
Oxygen bath, 296, 297 


THE HYDROUS OXIDES 


P 


Paints (see Pigments). 
antifouling, use of anhydrous 
cuprous oxide in, 146 
dryer for, anhydrous cobalt oxide 
as, 151 
manganese dioxide as, 302 
vanadium pentoxide as, 271 
Palladium dioxide, hydrous, 308 
monoxide, hydrous, 308 
sols as therapeutic agent, 308 
sesquioxide, hydrous, 308 
Paper, sizing of, silicate of soda in, 
198 , 
Péan de St. Gilles’ sol, 38-42, 58, 
283, 291 
Peptization, formation of sols by, 
82-87, 118-120, 130, 194, 215, 
225, 243, 244, 264 
Perchloric acid, adsorption by zir- 
conium dioxide, 245 
Petroleum refining, silica gel in, 
189, 190 
Phosphate, adsorption of, 405-406 
Pigments, beryllium oxide, 163 


ceramic, bismuth trioxide as, 
278 
cobalt or Renneman’s green, 
150 


or Thenard’s blue, 150 
cupric oxide, 141 
cuprous oxide, 146 
‘nickel oxide, 153 

chrome green, 80 
ferric oxide, 71, 73 
in bricks, 71, 73 

Guignet’s green, 76, 80, 81 
hematite as, 70 
magnetite as, 71 
Mars, 71, 73 
titanium dioxide, 236 
zine oxide, 172 
zirconia, 245 

Plaster of Paris, 382 

Plasticity in soils, 409 


SUBJECT INDEX 


Platinum dioxide, hydrous, 312, 313 
monoxide, 311 
sesquioxide, hydrous, 312 
sol, 62 
trioxide, 313 

Pleochroism, in cobaltous hydroxide, 

149 
streaming, in vanadium pentox- 
ide, 269 
Polymorphorism of crystalline lead 
monoxide, 228 

Porcelain, stains for (see Pigments, 

ceramic). 

Portland cement (see Cement). 

Potassium, adsorption of, 405 

Pozzolana, 383 

Praseodymium oxide, hydrous, 259 
peroxide, hydrous, 259 

Precipitating ions, adsorption of, 62, 

122-125 
Precipitation of sols by salt pairs, 
factors determining, 97—100 
values of electrolytes, 55-65, 91, 
223, 231, 270 
factors determining effect of 
concentration on, 60-62 
of ferric oxide sol, 56-58 
relation between and adsorp- 
tion of precipitating ions, 
122-125 
Procter-Wilson theory of swelling of 
gelatin, 19-22 

Prussian blue, 57, 59 

Purple of Cassius, 218, 219 
purples related to, 219, 220 


R 


Radium rays, effect on ceric oxide 
sol, 256, 257 

Rare earths, hydrous oxides of, 252- 
262 | 

Red lead or minium, 231, 232 


Refraction, streaming double, 54, — 


266-269 
Refractory, thorium dioxide as, 250 
zirconia as, 240 
Renneman’s green, 151 


447 


Rhodium dioxide, hydrous, 307, 308 
sesquioxide, hydrous, 307 
sol, 307 
Rhythmic 
rings). 
solution, 168 
Rubber, vulcanized. India, swelling 
of, 16 
zine oxide in, 172 
Rubies, artificial, 111, 112 
Ruthenium dioxide, hydrous, 306 
sol, 306 
oxide, hydrous, 305 
pentoxide, hydrous, 306, 307 
sesquioxide, hydrous, 305, 306 
tetroxide, anhydrous, 307 


Ss 


bands (see Liesegang 


Samarium oxide, hydrous, 261 
peroxide, hydrous, 261 
Sapphires (see Artificial and corun- 
dum gems). 
Scandium oxide, hydrous, 259, 260 
ageing, 259 
jelly, 260 
sol, 260 
Schulze’s law, qualitative nature of, 
124, 127 
Selenium sol, 62 
Setting of magnesia cement, 166 
of Portland cement, 389-393 
Silica gel (see Silicon dioxide, 
hydrous). 
Silica tanning, 333, 334 
Silicate of soda, 196-199 
applications, as an adhesive, 196, 
198 
cement for glass, 199 
deflocculating action, 199° 
detergent properties, 199 
preservation of eggs, 199 
printing and dye industry, 199 
sizing of paper, 198 
effect of brine, 197 
nature of commercial, 196 
preparation, 196 
viscosity, 197 


448 


Silicon dioxide, hydrous, 30, 31, 175- 
196, 200, 273, 334, 361, 362, 
377 

adsorption, of calcium, 403-405 
of gases, 177, 181-184 
of liquids from solution, 184- 
188 
isotherms for, 185, 186 
mechanism of, 187 
of phosphate, 403-405 
of solids from solution, 188, 
189. 
ageing, 179, 194 
applications, 189-192 
as catalyst, 192 
as filtering agent, 192 
fixation of nitrogen, 191 
in manufacture of sulfuric acid, 
191 
in petroleum refining, 189, 190 
in recovery of benzene from coal 
gas, 190, 191 
in vacuum refrigeration process, 
191 
composition, 175, 179 
effect of conditions of formation, 
178 
dehydration, 176, 177 
hysteresis in, 177 
elasticity, 180 
formation, 181, 192, 194-195 
of crystals in, 181 
gelatinous precipitate, 13 
heat of wetting, 184 
improved, 192 
jellies, 5, 6, 8, 11, 12, 175-193, 
197 
magnetic analysis, 179 
rhythmic bands in, 181 
sols, 62, 193-196 
as protective colloid for emul- 


sions, 195 
in treatment of tuberculosis, 
195 
structure, 179-182 


vibration, 180 
x-ray analysis, 179 


THE HYDROUS OXIDES 


Silk, adsorption of molybdenum 
blue by, 286 
mordants for, 344, 345, 350, 351, 
353, 354 
weighting of, with tin mordant, 
353, 354 
Silver oxide, hydrous, 156 
sol, 156 
Smoke screens, with titanium 


tetrachloride, 235, 236 
Soap, 287, 366 
jellies, 8, 10 
solutions, streaming double re- 
fraction, 268 
Sodium ‘‘protalbinate’’ as protec- 
tive colloid, 87, 142, 174, 278, 
298, 303 
Soil, 396-418 
acidity, 409-414 
role of soil colloids in, 409-414 
Soil colloids, 396-418 
adsorption of ammonia gas, 401; 
402 
bacteria in, 401 
color, effect of humic acid on, 
401 
composition, 396—402 
determination, 397, 398 
relation between composition 
and properties, 401, 402 
flocculation and _ deflocculation, 
414-418 
effect of bases, 416, 417 
formation, 399 
heat of wetting, 401, 402 
humic acid, 400 
organic, 400 
role of, 403-414 
in adsorption of salts, 403-407 
of water, 407-408 
in plasticity, 409 
in soil acidity, 409-414 
complex-acid theory, 412-414 
selective-adsorption theory, 
409-411 


Sol-gel transformation, 9, 11, 16, 66, 


121 


SUBJECT INDEX 


Solid-solution theory of jelly struc- 
ture, 4 
Sorel’s magnesia cement, 166 
Spectrum, reflection, of neodymium 
oxide, 261 
Sponge or micellar theory of jelly 
structure, 8, 12 
Stannic acid, meta and ortho (see 
Stannic oxide, hydrous). 
non-existence of, 201, 211-214 
Stannic oxide, hydrous, 30, 200, 202— 
225, 233, 273, 334, 338, 356 
action of acids, 205-209 
of alkalies, 209, 210 
adsorption by silk, 254 
of dyes (see Mordants). 
of hydrochloric acid by, 206 
of hydrous ferric oxide, 220- 
222 
of phosphoric acid, 208, 209 
of potassium ferrocyanide, 216 
of stannic chloride, 217 
ageing, 207 
in presence of nitric acid, 208 
alleged forms, 203 
question of isomers, 211-215 
relationships between, 204 
relative peptizability of, 213, 
214 
composition, 204, 205 
absence of hydrates, 205 
complex theory of, 207 
jellies, 11, 223, 224 
magnetic analysis, 214 
mordant for cotton, silk, and wool, 
210, 352-354 
mutual adsorption of and other 
hydrous oxides, 221, 222 
pepization of, by alkalies, 210 
by ferric nitrate, 222 
by hydrochloric acid, 207 
by nitric acid, 208, 213 
by sulfuric acid, 208 
by washing, 225 
sols, 62, 96, 215-223 
ageing, 216-218 
effect of tartaric acid on, 217 


449 


Stannic oxide, hydrous, sols, behav- 
ior with colloidal metals, 
218-220 
with other hydrous oxides, 
220-223 
formation, 215 
purple of Cassius, 218, 219 
x-ray analysis, 11, 214 
Stannous oxide, hydrous, 224, 225 
Starch, sol, 221, 298 
swelling of, 16 
Starch-iodine, adsorption by zir- 
conium dioxide, 244 
Sterilization in purification of water, 
367, 370 ; 
Structure of gels, 3-15 
Strychnine nitrate, adsorption of, 331 
Sulfur in tanning, 334 
sol, 94 
Sulfuric acid, silica gel in manufac- 
ture, 191 
Swelling, albumin, 16 
application of Donnan theory of 
membrane equilibria, 19—22 
effect of hydrogen ion concentra- 
tion on, 17-22 
of neutral salts on, 21 
fibrin, 16 
gelatin, 16-23 
preparation of gels by, 16 
pressure of jellies, 32, 33 
Procter- Wilson theory of, 19-22 
reversibility of, 22 
starch, 16 
vulcanized india rubber, 16 


it 


Tannin, 287, 315-317, 354-357 
adsorption of, by hydrous alum- 
ina, 316, 357 
cotton, 354 
gelatin, 316 
hide, 316-320 
magenta, 365 
wool, 355 
mordant, 354-356 
fixing of, 356 


450 


Tanning, 314-335 
agents, miscellaneous, 334, 335 
alumina, 333 
ceric oxide, 254 
chrome, 323-332 
iron, 333 
mineral, 315, 322-335 
adsorption theory of, 325, 328 
chemical theory, 329, 330 
criticism, 330-332 
preparation of hide for, 314 
silica, 333, 334 
vegetable, 315-322 
adsorption theory, 316, 317, 320 
chemical interpretation, 321, 
322 
Procter- Wilson theory, 318, 319 
quinones in, 322 
with bromine, 334 
chlorine, 334 
formaldehyde, 322 
insoluble powders, 334 
sulfur, 334 
Tantalum pentoxide, hydrous, 273, 
274 
glow phenomenon, 273 
separation from columbium pent- 
oxide, 272 
sols, 274 
vapor-tension isotherms, 273, 274 
Terbium oxide, hydrous, 261, 262 
Thallic oxide, hydrous, 132, 183, 337 
Thallous hydroxide, 133 
Thenard’s blue, 150 
Thorium dioxide, 246-250 
catalyst, 250 
for gastro intestinal diagnosis, 250 
hydrous, 246-250, 285, 337, 365 
ageing, 247, 248 
adsorption of Congo red by, 364 
sols, 246, 247 
in Welsbach mantel, 249 
refractory, 250 
Thorium peroxide, hydrous, 250, 251 
Thulium oxide, hydrous, 262 
Tin mordants, 352-354 
weighing of silk with, 3538, 354 


THE HYDROUS OXIDES 


Titanic acids, meta and ortho (see 
Titanium dioxide, hydrous). 
Titanium dioxide, 236 
hydrous, 2383-236, 338 
adsorption of dyes, 234 
ageing, 234 
alleged forms, 233, 234 
glow phenomenon, 233, 234 
jellies, 235 
mordant, 235 
sol, 234, 235 
in corundum gems, 112 
in paints, 236 
monoxide, 236 
peroxide, hydrous, 237 
adsorption of salts by, 237 
sesquioxide, hydrous, 236 
tetrachloride, in producing smoke 
screens, 235, 236 
Topaz, oriental, artificial, 111 
Tuberculin, adsorption by hydrous 
alumina, 128 
Tuberculosis, calcification in, 196 
silica gel in treatment of, 195 
Tungsten blue sol, 62, 292 
as dye bath, 292 
color, 292 
trioxide, anhydrous, 288 
hydrous, white, 288-291 
dehydration curve, 288, 289 
protective action of molyb- 
dium trioxide, 290 
sol, 269, 289-291 
monohydrate, yellow, 31, 288, 
289 
dehydration curve, 288, 289 
Turgite, color of, 70 


U 


Uranium dioxide, anhydrous, 293 
catalyst for synthesis of formal- 
dehyde, 293 . 
hydrous, 293, 337 
trioxide, dihydrate, 292, 293 
color, 293 
sol, 292, 293 


SUBJECT INDEX 


V 


Vanadium bronze, 270 
dioxide, hydrous, 271 
pentoxide, hydrous, 263-271 
as catalyst, 270 
color, 265, 270 
dryer for linseed oil, 271 
jelly, 269, 270 
mordant, 271 
sol, 267-270 
dielectric constant, effect of 
ageing on, 268 
elasticity, 267 
Majorana phenomenon, 565, 
266-269 
precipitating action of eclec- 
trolytes, 269, 270 
preparation, 264, 265 © 
streaming double refraction, 
266-269 
streaming pleochroism, 269 
x-ray analysis, 268 
sesquioxide, hydrous, 271 
Vapor pressure of gels, 30-33 
Vegetable tanning (see Tanning). 
Vegetation, artificial, 197 
Vibration in silica gel, 80 
Victoria blue, adsorption of, 331 
Viscosity-time curves of ceric oxide 
sol, 255, 256 
von Schroeder’s paradox, 31 
Vortex rings with manganese dioxide 
sol, 297 


W 


Water glass (see Silicate of soda). 
Water purification, 366-381 
aeration in, 369 
alum in, 370-880 
alumina floc in, 371-380 
composition, 378-380 
formation, effect of anion on, 
375, 376 
effect of colloidal matter on, 
379 


451 


Water purification, alumina floc in, 
formation, effect of colloi- 
dal silica, 377 
effect of hydrogen ion concen- 
tration on, 372, 373 
effect of hydrogen ion con- 
centration on time for, 372, 
373 
effect of mechanical circula- 
tion on time for, 373 
optimum conditions for obtain- 
ing, 380 
by disinfection, 367, 381 
by filtration, 367-380 
mechanical, 369-381 
applicability of, 370 
coagulents in, 370-381 
slow sand, 267-369 
applicability of, 370 
by sterilization, 367, 370 
coagulents in, 370-381 
color, removal of, aluminum ion 
in, 370, 371, 378 
ferric oxide floc in, 380, 381 
optimum conditions for forming, 
381 
optimum conditions for successful, 
378 
use of colloidal alumina, 377 
Welsbach .mantel, 163, 249, 250, 
254 
theory of, 249, 250 
White lead, 229 
composition, 229 
Wool, adsorption of hydrous alu- 
mina by, 340 
mordants for, 339-344, 347-353 
of sulfuric acid by, 341 


x 


Xanthosiderite, 35 
X-ray analysis of aluminum oxide 
trihydrate, 109, 110 
beryllium oxide, 159 
ceric oxide, 253 
cobaltous hydroxide, 148 


452 


X-ray analysis of cupric oxide, 253 
ferric oxide, 11 
gelatin, 11 
lead monoxide, 228 
magnesium hydroxide, 164 
molybdenum trioxide, 280 
silicon dioxide, hydrous, 11, 179 
stannic oxide, 11, 214 
thorium dioxide, 247 
vanadium pentoxide, 268 
zirconium dioxide, 239 


Y 


Ytterbium oxide, hydrous, 262 
Yttrium oxide, hydrous, 252, 334, 
sol, 262 


Z 


Zine hydroxide, 171 
sol, 171 
Zine oxide, anhydrous, antiseptic 
action of, 172 
color, 253, 254 
use in adhesive tape, 172 


THE HYDROUS OXIDES 


Zinc oxide, anhydrous, use in dental 
cements, 172 
enamel pigment, 172 
paints, 172 
rubber, 172 
hydrous, 169-172, 337 
action of alkalies,.170, 171 
ageing, 169 
color, 253 
Zirconia, uses of, 240-241 
Zirconium dioxide, hydrous, 237- 
241, 285, 337, 338, 365 
adsorption by, 244-245, 364 
ageing, 160, 238, 239 
alleged form, relationship 
between, 239, 240 
glow phenomenon, 239 
jelly, 248, 244 
mordant, 245 
sols, 241-244 
by hydrolysis of zirconium 
salts, 241-243 
by peptization, 243, 244 
x-ray analysis, 238 
peroxide, hydrous, 245, 246 














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